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

Small groups of protesters held rival demonstrations on Monday as judges inside the Supreme Court in London were hearing arguments as to whether the government had the right to leave Europe without parliamentary approval.

Britain's Supreme Court began hearing a landmark case on Monday that will decide who has the power to trigger the UK's exit from the European Union - the government or Parliament.

The legal battle has major constitutional implications for the balance of power between the legislature and the executive, and has inflamed Britain's already raw wound over how and whether to leave the EU.

Prime Minister Theresa May plans to trigger Article 50 of the EU's key treaty by the end of March, 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.

London - 5 December 2016

1. Pro-Europe demonstration in front of Supreme Court

2. Tilt down of sign reading (English): "Stop Brexit fight racism and the scapegoating of immigrants. Defend freedom of movement."

3. Various of pro-Europe protesters chanting, holding banners

4. Pan from police to protesters in front of Supreme Court

5. Rival protest groups in front of Supreme Court, police officers at scene

6. Pro-Europe protesters shouting (English): "Brexit is racist, stop Brexit now!"

7. Smart phone screen showing protesters

8. Rival protesters discussing

9. Pro-Brexit protesters, woman waving Union flag

10. Man being led away by police

11. Policewoman

12. Supreme Court and protesters

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

A law professor said an appeal of the High Court ruling stating that the British prime minister can't trigger the UK's exit from the European Union without parliamentary approval was unlikely to succeed.

Jeff King, of University College London, said Thursday's comprehensive ruling on an expansive set of grounds could cause the government's argument to fail.

But King cautioned that those hoping for a reversal of the referendum result would likely be disappointed.

London - 3 November 2016

1. Various set up shots of Professor of Law Jeff King, University College London

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

"It was very clear, there are many points that rather than just the courts saying that we don't need to rule on this point, the court rather said that we think that point is completely untenable, in some cases suggesting that an argument was divorced from reality. So a court that was very divided on this question might well have given a much slimmer narrower ruling based on very few grounds instead of an expansive set of grounds, dealing with only a few crucial points instead of dealing with many more subtle points, but it chose the latter route. In so it gave a comprehensive ruling on many points, many of which could cause the government's argument to fail. And that gives me confidence that an appeal would be unlikely to succeed under the circumstances."

3. Cutaway of King

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

"It may feel that legally there are arguments that it wishes either to make again or to bring new arguments to the attention of the Supreme Court. It may feel that the political consequences of not pursuing the appeal could be problematic, it might be accused of not having followed up every available avenue. These type of accusations are already being made about the pleadings of the government in this very case. So for those reasons, it may have good political and legal reasons to appeal. My sense is that the appeal would be unlikely to be successful. I think that Supreme Court would have even more confidence to be as unequivocal as this court has been."

5. Various of King

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

"If that 48 percent (those supporting remain) is hoping this legal challenge will stop Brexit, I think they will be disappointed. I think whether Brexit goes ahead will be a function of where Parliament takes this issue from here. And that's exactly what all the judges in the case were taking pains to point out.''

7. Cutaway of King

Britain's government must get parliamentary approval before starting the process of leaving the European Union, the Supreme Court ruled on Tuesday.

The decision potentially delays Prime Minister Theresa May's plans to trigger exit negotiations by the end of March.

London - 24 January 2017

1. Various of Parliament

2. Evening Standard newspaper vendor talking to security officer

3. Close of Evening Standard newspaper, reading (English) "Judges' blow to May over Brexit."

4. Vendor and security officer talking

5. Close of newspaper lead, reading (English) "PM can't trigger Article 50 without Parliament backing, says supreme court in historic ruling."