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