Opposing groups of campaigners gathered outside the UK Supreme Court in London on Monday as judges started hearing the government's appeal against a ruling that Parliament must hold a vote before Britain's European Union exit negotiations can begin.

Prime Minister Theresa May plans to trigger two years of divorce talks before the end of March by invoking Article 50 of the EU's key treaty, using centuries-old government powers known as royal prerogative.

Those powers - traditionally held by the monarch but now used by politicians - allow the government to join or leave international treaties without consulting lawmakers.

Financial entrepreneur Gina Miller and hairdresser Deir Dos Santos went to court to argue that leaving the EU will remove some of their rights, including free movement within the 28-nation bloc, and that shouldn't be done without Parliament's approval.

Last month, three High Court judges agreed - but the government says they have misinterpreted the law.

The case has raised a constitutional quandary and inflamed the country's heated debate about Brexit.

November's ruling infuriated pro-Brexit campaigners, who saw the lawsuit as an attempt to block or delay Britain's EU exit.

Miller says she has received abuse and death threats.

She arrived at court Monday with her lawyers, greeted by cheers from pro-EU campaigners dressed as judges atop an open-topped double-decker bus.

A banner on the side read "In democracy, Leave hypocrisy."

"Nigel, where are you?" they sang mockingly, in reference to former UK Independence Party leader Nigel Farage, a prominent voice in the leave campaign.

He had promised to lead a march on the Supreme Court to demand judges respect the will of the majority.

It was cancelled last week after organisers said there was a risk it could be hijacked by far-right extremists.

Two anti-EU protesters held placards, one calling the case an "establishment stitch-up."

Many legal experts say the government will likely lose its appeal and be forced to give Parliament a vote.

London - 5 December 2016

1. Exterior of UK Supreme Court

2. Close up of European Union flag

3. Police entering Supreme Court

4. Wide of pro-Remain protesters from Britain for Europe singing as they arrive in open-topped London bus UPSOUND (English) "Nigel, where are you?"

5. Various of protesters outside court building, holding EU and UK flags

6. SOUNDBITE (English) David Gullant, Protester from Surrey branch of Britain for Europe:

"I am a British citizen, but I want to make sure that the UK due process is followed. So this is not about Brexit pros and cons, it's about making sure that if you have an Act of Parliament, only Parliament can change that Act of Parliament. It is not up to the prime minister to overturn it, it has got to go through due process."

7. Pro-Brexit protester Julia Waller

8. SOUNDBITE (English) Julia Waller, pro Brexit protester

"Well, I hope that the government, I hope Theresa May's hand, is strengthened. We've got to give her some credit - she can't expose her hand to Europe. Europe doesn't have our best interests at heart anyway, and I think we want to be free, and when people voted they didn't just vote for a soft Brexit or a grey Brexit, those words didn't exist then. We just voted for Brexit. We want to get out - end of (discussion)."

9. Pan from pro-Europe protesters to Supreme Court

As UK's Prime Minister Theresa May wants to start Britain's divorce from the European Union (EU), eleven judges may stand in her way.

On Monday, May's government will ask the justices of the Supreme Court to overturn a ruling that Parliament must hold a vote before Britain's exit negotiations can begin, a case that has raised a constitutional quandary and inflamed the country's heated debate about Brexit.

May plans to trigger two years of divorce talks before the end of March 2017 by invoking Article 50 of the EU's key treaty, using centuries-old government powers known as royal prerogative.

The powers - traditionally held by the monarch but now used by politicians - enable decisions about joining or leaving international treaties to be made without a vote of Parliament.

A hairdresser and a financial entrepreneur, Deir Dos Santos and Gina Miller, went to court to argue that leaving the EU will remove some of their rights, including free movement within the bloc, and that shouldn't be done without Parliament's approval.

Last month, three High Court judges agreed, but the government says they have misinterpreted the law.

Government lawyers led by Attorney General Jeremy Wright will argue this week that the British people have spoken by voting in a June 23 referendum to leave the EU.

In written arguments published before Monday's hearing, government lawyers say the use of royal prerogative to start the talks is "constitutionally appropriate and lawful."

Many legal experts say the government will likely lose its appeal and be forced to give Parliament a vote, but the government was left with little choice than to proceed with the case anyway.

"It's politically problematic, I think, to be seen to not pursue all legal avenues to obstruct this attempt to, in the eyes of some critics, derail the Brexit process," says Jeff King, Professor of Law, University College London.

In a reflection of the constitutional importance of the case - which hinges on the balance of power between Parliament and the executive - all 11 Supreme Court judges will hear the appeal, the first time the full court has sat since it was founded in 2009.

The case is scheduled to last four days, with a ruling likely in January.

Complicating the picture are new participants including the Scottish government, which argues the Edinburgh-based Scottish Parliament should get a vote, too.

Britons voted by 52 to 48 percent on June 23 to leave the EU, but voters in Scotland strongly backed staying in, and the Scottish government says they should not be dragged out of the EU against their will.

London - 1 December 2016

1. Exterior of UK Supreme Court building, traffic on street

2. Close of symbol reading (English): "The Supreme Court"

3. Various exteriors of Supreme Court building

4. Various of Jeff King, Professor of Law, University College London, looking at book

5. SOUNDBITE (English) Jeff King, Professor of Law, University College London:

"Well, I think that there are two reasons probably (why the government appealed the High Court ruling that Parliament must hold a vote before negotiations of Britain's exit from the European Union can begin). The first is that it was very confident that it would win at the beginning, and I think it's reluctant to move from that position of confidence. And secondly, it's politically problematic, I think, to be seen to not pursue all legal avenues to obstruct this attempt to, in the eyes of some critics, derail the Brexit process."

6. Cutaway of King

7. SOUNDBITE (English) Jeff King, Professor of Law, University College London:

"The reaction in some of the media (to the government's loss in the High Court), in particular in the Daily Mail and the Daily Express was, I would say, outrageous. It has been common to criticise judges in the past and I think criticism of judges is perfectly acceptable - they are public officials after all, but this type of language, 'Enemies of the people' (Daily Mail headline the day after the ruling) reminded me of the kinds of things we saw in Nazi Germany."

8. Cutaway of King

9. SOUNDBITE (English) Jeff King, Professor of Law, University College London:

"This is quintessentially a role for judges to play. It's an unclear legal position. The devolved governments don't agree, many members of parliament don't agree. There needs to be an authoritative ruling on what the role of these statutes is in the Brexit process, and that can only be given by a court."

10. Cutaway of King

11. SOUNDBITE (English) Jeff King, Professor of Law, University College London:

"I think that a government loss at the Supreme Court would not derail the Brexit process, but I also think it would be more than a mere speed bump because having to bring forward legislation to give notice would mean that there would be a participation of a much broader range of voices, both in Parliament and in the devolved legislatures and among the devolved governments. And when these voices are heard there is a political obligation of sorts to respond to these voices, to take them into account, to not proceed in a way that ignores them altogether, and I think that will make a difference to the outcome of the Brexit process, but I don't think it will stop it."

12. Cutaway of King

13. Pan of books

14. Various exteriors of Houses of Parliament

Britain's government must get parliamentary approval before starting the process of leaving the European Union, the Supreme Court ruled on Tuesday, potentially delaying Prime Minister Theresa May's plans to trigger exit negotiations by the end of March.

The 8-3 ruling from the judges forces the government to put a bill before Parliament, giving pro-EU politicians a chance to soften the terms of Brexit - Britain's exit from the EU.

"Leave" campaigners had objected, saying Parliament shouldn't have the power to overrule the electorate, which voted to leave the bloc in a June 23 referendum.

The Supreme Court's decision doesn't mean that Britain will remain in the EU - but it could delay the process.

Still, May's Brexit secretary, David Davis, told the House of Commons on Tuesday that the government timetable remained on track, and that officials would introduce a bill within days.

"They (parliament) need to be involved before we leave," said Charlie Mullins, founder of Pimlico Plumbers, who helped fund Article 50 legal challenge.

May had argued the referendum gave her a mandate to take Britain out of the EU and that discussing the details of her strategy with Parliament would weaken the government's negotiating position.

UKIP leader Paul Nuttall said the result was what he expected, saying "I think we've lost the battle today and I'm disappointed but not surprised, but we will win the war."

London - 24 January 2017

1. Pan down UK Supreme Court entrance

2. Police outside

3. Top shot media and entrance

4. Set up of Charlie Mullins, founder of Pimlico Plumbers

5. Close up Mullins' tie

6. SOUNDBITE (English) Charlie Mullins, founder of Pimlico Plumbers, helped fund Article 50 legal challenge:

"Well I think it was all unnecessary. If she would have got parliament involved, rather than you know, saying that she's gonna get on with it, then she was saying, 'oh, they can vote on this once she's negotiated a deal' - well that's all too late then. Once she's negotiated a deal, once we've left the EU and we're offered the deal we've got, there is no point in parliament getting involved. They need to be involved before we leave. And now I think they can put enough pressure on to her to make sure we get a soft Brexit."

7. Pan down Supreme Court

8. Various pro-EU demonstrators

9. Various exteriors of UK parliament

10. Various cutaways of Paul Nuttall, UK Independence Party leader

11. SOUNDBITE (English) Paul Nuttall, UKIP leader:

"I think it's what we expected, I think we've lost the battle today and I'm disappointed but not surprised, but we will win the war. And now these guys behind me in the House of Commons and the House of Lords have a duty to enact the will of the people and I want to make it clear what that will is. People voted on mass on June 23rd to leave the European Union. They also voted the leave the single market. The Prime Minister of the day, David Cameron, the Chancellor, the Foreign Secretary of the day, Philip Hammond, they all made it clear, leaving the EU means leaving the single market. The British people went out and did that by a score of 52 percent. So leaving must mean leaving."

12. Mid of Parliament

13. Various set ups Andrew Blick, Director History and Policy organisation and Lecturer King's College London

14. SOUNDBITE (English) Andrew Blick, Director History and Policy and Lecturer King's College London:

"The courts upholding that principal is a very important constitutional moment that no doubt will appear in all the textbooks for years to come. No question about the constitutional significance. The political significance is a bit more complicated. What political difference will it really make? It seems unlikely that the Commons, the House of Commons, the elected chamber of parliament, would want to be seen to be blocking Brexit, to be seen to be resisting the will of the people as expressed in a referendum. That seems very unlikely. There is perhaps an outside chance that the House of Lords, the unelected chamber, might offer some kind of resistance to the bill."

15. Cutaway

16. SOUNDBITE (English) Andrew Blick, Director History and Policy and Lecturer King's College London:

"I think that timetable (set out by British Prime Minister Theresa May) still looks very plausible, but the consequence of this judgment today is that the matter is slightly out of her (May) hands in that now she has to get statutory authorisation, she has to take a bill through parliament, she has to follow through those parliamentary procedures. It can be done quickly but it does require both Houses of Parliament to agree to it if she wants it happen quickly, so there is a slight variable in there, that wasn't in there before."

17. Various exteriors of Parliament

The lead plaintiff in the British Supreme Court case said on Tuesday that she was happy with the ruling because it provides the legal foundation to trigger Brexit.

Gina Miller spoke shortly after the Supreme Court announced in a majority 8-3 decision that the government must get parliamentary approval before starting the process of leaving the European Union.

Miller also said that the case was about the legal process, not politics.

British Attorney General Jeremy Wright said that the government will comply with the ruling, and that a statement will be made in Parliament later.

The court also unanimously decided that there's no need to consult Scotland, Wales, Northern Ireland on Brexit.

Tuesday's ruling raises hopes among pro-EU politicians that they will be able soften the terms of the U.K.'s withdrawal from the bloc.

British Prime Minister Theresa May wanted to use centuries-old powers known as royal prerogative to invoke Article 50 of the EU treaty and launch two years of exit talks.

++AUDIO QUALITY AS INCOMING++

UK POOL

London - 24 January 2017

1. Lead plaintiff Gina Miller exiting court

2. SOUNDBITE (English) Gina Miller, Lead plaintiff: ++AUDIO AS INCOMING++

"Good morning, today eight of the 11 supreme judges upheld the judgment handed down by the High Court in November, in a case that went to the very heart of our Constitution and how we are governed. Only parliament can grant rights to the people and only parliament can take them away. No prime minister, no government, can expect to be unanswerable or unchallenged. Parliament alone is sovereign. This ruling today means that MPs we have elected will rightfully have the opportunity to bring their invaluable experience and expertise to bear in helping the government select the best course in the forthcoming Brexit negotiations, negotiations that will frame our place in the world and all our destinies to come."

++BLACK FRAMES++

3. SOUNDBITE (English) Gina Miller, Lead plaintiff:

"There is no doubt that Brexit is the most divisive issue of a generation, but this case was about the legal process, not politics. Today's decision has created legal certainty based on our democratic process and provides the legal foundations for the government to trigger Article 50."

++ENDS ON SOUNDBITE++

Britain's Supreme Court will rule on Tuesday on whether the prime minister or Parliament has the right to trigger the process of taking Britain out of the European Union.

The 11 justices will either uphold an earlier ruling giving Parliament a direct role in invoking Article 50 of the EU's Lisbon Treaty or reject that ruling in favour of the government's claim it can do so without a vote in Parliament.

Article 50, which has never been used before, starts the formal process of taking Britain out of the 28-nation EU.

The legal battle follows a successful High Court action led by businesswoman Gina Miller.

A reversal of the lower court ruling would give Prime Minister Theresa May the authority to start Article 50 negotiations when she chooses without input from Parliament.

She has said, however, that Parliament will be asked to approve any Brexit deal reached at the conclusion of the talks.

UK POOL

London - 24 January 2017

1. Lord Richard Keen QC, Advocate General for Scotland, (wearing red scarf) and Jeremy Wright UK Attorney General arriving at Supreme Court

2. Various of lead claimant Gina Miller arriving, posing for cameras

3. Various of more arrivals

4. Protester with banner being led away by police

5. Police arriving

UK Supreme Court judges were heading in for a third day of a four-day hearing as they considered whether government or Parliament has the power to lead the country out of the European Union.

Prime Minister Theresa May plans to launch EU exit talks by the end of March using ancient powers known as royal prerogative, which enable decisions about joining or leaving international treaties to be made without a parliamentary vote.

Several claimants went to court to argue that leaving the EU would remove some of their rights, including free movement within the bloc, and that such an action could not be taken without legislation in Parliament.

Last month the High Court agreed that the government cannot trigger Article 50 of the EU's key treaty, launching two years of exit negotiations, without lawmakers' support.

The government is challenging that decision at the country's top court - leaving the 11 justices to rule on the balance of power between the legislature and the executive.

CORRECTION - CLIENTS PLEASE NOTE THE MAN IN SHOT 1 IS ACTUALLY CLAIMANT CHARLIE MULLINS OF PIMLICO PLUMBERS, NOT HAIRDRESSER DEIR DOS SANTOS AS ORIGINALLY STATED

POOL - AP CLIENTS ONLY

London - 7 December 2016

1. Various of entrepreneur Charlie Mullins of Pimlico Plumbers arriving at Supreme Court. Mullins, the boss of London-based Pimlico Plumbers is one of a group of business people helping to fund the court challenge

UK Supreme Court judges were heading in for a third day of a four-day hearing as they considered whether government or Parliament has the power to lead the country out of the European Union.

Prime Minister Theresa May plans to launch EU exit talks by the end of March using ancient powers known as royal prerogative, which enable decisions about joining or leaving international treaties to be made without a parliamentary vote.

Several claimants went to court to argue that leaving the EU would remove some of their rights, including free movement within the bloc, and that such an action could not be taken without legislation in Parliament.

Last month the High Court agreed that the government cannot trigger Article 50 of the EU's key treaty, launching two years of exit negotiations, without lawmakers' support.

The government is challenging that decision at the country's top court - leaving the 11 justices to rule on the balance of power between the legislature and the executive.

POOL

London - 7 December 2016

1. Lead claimant Gina Miller and team arriving at Supreme Court

2. Lord Richard Keen QC, Advocate General for Scotland, and Jeremy Wright UK Attorney General arriving

Proceedings began on Monday at the UK's highest court as the British government attempts to overturn a ruling that Parliament must hold a vote before Britain's European Union exit negotiations can begin.

Prime Minister Theresa May plans to trigger two years of divorce talks before the end of March by invoking Article 50 of the EU's key treaty, using centuries-old government powers known as royal prerogative.

The powers - traditionally held by the monarch but now used by politicians - enable decisions about joining or leaving international treaties to be made without a parliamentary vote.

Financial entrepreneur Gina Miller and hairdresser Deir Dos Santos went to court to argue that leaving the EU will remove some of their rights, including free movement within the bloc, and that shouldn't be done without Parliament's approval.

Last month, three High Court judges agreed.

But the government says they have misinterpreted the law.

Government lawyers led by Attorney General Jeremy Wright will argue this week that the British people have spoken by voting in a June 23 referendum to leave the EU.

They say a parliamentary vote would mean getting lawmakers "to answer precisely the same question which was put by Parliament to the electorate and has been answered in the referendum."

Many legal experts say the government will likely lose its appeal and be forced to give Parliament a vote.

In a reflection of the constitutional importance of the case - which hinges on the balance of power between Parliament and the executive - all 11 Supreme Court judges will hear the appeal, the first time the full court has sat since it was founded in 2009.

The case is scheduled to last four days, with a ruling likely in January.

UK POOL

London - 5 December 2016

1. Zoom in from man waving European Union flag outside court building to flag

2. Attorney General for England and Wales Jeremy Wright arriving with his associates

3. Protest bus passing outside court, people on it chanting UPSOUND (English) "Nigel, where are you!"

4. Gina Miller, investment fund manager and philanthropist who was one of the team that won the High Court ruling, arriving with her associates, posing for photographers in front of court entrance