By Michael Holden and Estelle Shirbon
LONDON (Reuters) - Prime Minister Theresa May must give parliament a vote before she can formally start Britain's exit from the European Union, the UK Supreme Court ruled on Tuesday, giving lawmakers who oppose her Brexit plans a shot at amending them.
A "straightforward" bill will now be rushed to parliament within days, the government said after the country's highest judicial body decided May could not use executive powers known as "royal prerogative" to invoke Article 50 of the EU's Lisbon Treaty and begin two years of divorce talks.
However, the judges did remove one major potential obstacle for the government, saying May did not need the approval of Britain's devolved assemblies in Scotland, Wales and Northern Ireland before triggering Brexit.
"We will within days introduce legislation to give the government the legal power to trigger Article 50," Brexit minister David Davis told parliament. "This will be the most straightforward bill possible to give effect to the decision of the people and respect the Supreme Court's judgment."
May has said she intends to invoke Article 50 before the end of March but the ruling means the Brexit process is now open to scrutiny from lawmakers, the majority of whom had wanted to stay in the EU.
However, the main opposition Labour Party has said it would not block Brexit although it would try to amend the legislation.
"Labour will seek to build in the principles of full, tariff-free access to the single market and maintenance of workers' rights and social and environmental protections," party leader Jeremy Corbyn said.
Media reports have suggested that up to 80 Labour lawmakers (MPs) in the 650-member House of Commons, the lower chamber, would ignore Corbyn and vote against triggering Article 50, while the small Liberal Democrat Party said it would oppose Brexit unless there was a second referendum on the final deal.
Meanwhile, the Scottish National Party, which has 54 MPs, vowed to put forward 50 "serious and substantive" amendments. However, the opponents of Brexit are still likely to be some way short of the numbers needed to either delay May's timetable or to stop it.
The upper chamber, the House of Lords, could also seek to amend the plans but ministers are confident that unelected peers would not try to stop Britain leaving the EU after voters backed Brexit by 52-48 percent in last June's referendum.
BREXIT PLANS UNCHANGED
May's spokesman said the court's decision did nothing to change the path of Brexit or her timetable.
Davis said: "The point of no return was passed on June 23rd last year. This judgment does not change the fact that the UK will be leaving the European Union."
Last week May set out her stall for negotiations, promising a clean break with the world's largest trading bloc as part of a 12-point plan to focus on global free trade deals, setting a course for a so-called "hard Brexit".
Some investors and those who backed the "remain" campaign hope that lawmakers, most of whom wanted to stay in the EU, will force May to seek a deal which prioritizes access to the European single market of 500 million people, or potentially even block Brexit altogether.
Sterling initially rose on the news that the government had lost its appeal, but it then fell over half a cent to hit day's lows against the dollar and euro after the ruling that the devolved assemblies did not need to give their assent.
Those who campaigned for Britain to leave the EU said the vote on triggering Brexit should be a mere formality.
"Any attempt to delay the Brexit process ... would be an unforgivable betrayal of the British people," said Richard Tice, co-chairman of the Leave Means Leave campaign. "The Lords should also follow suit; any delay by them would ensure their abolition."
While Tuesday's ruling has settled the argument over the role of parliament in starting the Brexit process, other hurdles and headaches await May.
Scotland's First Minister Nicola Sturgeon said the court's decision on devolved assembles raised the specter of another Scottish independence referendum because Scots, who voted in favor of staying in the EU, were not being treated as equal partners.
May's spokesman also stressed that the government's assertion that Article 50 was irreversible, but another legal case is being prepared to challenge that view in the courts.
(Additional reporting by William James, Kylie MacLellan, Elizabeth Piper and Alistair Smout; Editing by Guy Faulconbridge, Pravin Char and Giles Elgood)