LONDON (AFP) - British Prime Minister Theresa May faces a landmark court ruling on Tuesday (Jan 24) that could put a dent in her Brexit plans by handing control of the process to restive lawmakers.
The Supreme Court will decide whether she can use her executive power to begin formal talks on leaving the EU, or whether she must seek prior approval from parliament.
The 11 judges are widely expected to back an earlier High Court ruling that the magnitude of Brexit means the process to instigate it can only be introduced through formal legislation.
May has promised to trigger Article 50 of the EU's Lisbon treaty, beginning two years of divorce talks, by the end of March.
In the event they lose the case, ministers are preparing to rush emergency legislation through the Houses of Commons and Lords.
Opponents would almost certainly table amendments to try to tie the government's hand in negotiations, for example on the rights of EU citizens already in Britain.
But the vote on Article 50 should pass, because although May has only a slim majority, the main opposition Labour party has agreed not to block it.
The Supreme Court ruling may yet create further complications, including whether the devolved governments in Scotland, Wales and Northern Ireland must approve May's plans.
And this week May promised parliament a vote on the final Brexit deal - raising the prospect, however remote, that MPs could reject it.
The original High Court decision in November drew outrage from Brexit supporters, who accused the claimants of trying to undo the result of last June's EU referendum.
One tabloid newspaper condemned the judges as "Enemies of the People" - and tensions are still running high.
But Jo Murkens, associate law professor at the London School of Economics, said the case was about the limits of the government's "royal prerogative" powers.
"It would be much easier if the prime minister could just do as she pleased using prerogative power. The problem is that the courts have not allowed that since the 17th century," he told AFP.
He said a ruling against the government would be unlikely to bind its hands, but would clarify "there is a proper process and it's (through) parliament".
"If you've got a majority then you're in the clear - but if you don't have a majority you've got a political problem," he told AFP.
The majority of MPs campaigned against Brexit, but most now accept it will happen - and that the process will begin within weeks.
May's announcement this week that she would pull Britain out of Europe's single market has galvanised some of her critics, however.
Dozens of Labour MPs may defy their leader Jeremy Corbyn and vote against the government.
The Scottish National Party (SNP), which has 54 MPs in the 650-seat House of Commons, is also firmly against Brexit.
But Tim Bale, professor of politics at Queen Mary University of London, said opposition among May's own MPs had all but disappeared in recent weeks.
"It's quite difficult to see her running into too much trouble now," he told AFP.
"Her biggest problem is not the people behind her or in front of her in the Commons. Her biggest problem is the 27 heads of government who she has to negotiate with." .
The SNP could cause more trouble if the court rules that the devolved nations must agree to start Brexit talks, as the party controls the government in Edinburgh.
The situation may also change if the economic outlook worsens, and public opinion turns against the government by the time MPs vote on a final deal.
Losing such a vote would likely force a general election, but if the EU refuses to agree to more negotiations, it could also see Britain leaving with no deal at all.
Some have argued that in such an eventuality, Britain could revoke its Article 50 notification, thereby cancelling Brexit.
"If parliament votes it down, then we're in no man's land," said Murkens.
Ministers refused to speculate on the future vote but Brexit minister David Davis said that Britain would be leaving the EU regardless.
"The vote will not change that," he said.