Britain’s high court dealt a blow to backers of Britain’s exit from the European Union, or Brexit, by ruling Thursday the government must consult Parliament before starting the disengagement.
Proponents of the decision, made by voters in June, say they will appeal.
Prime Minister Theresa May has said she will trigger Article 50 of the Lisbon treaty in March to begin the process of disengagement and start negotiations to end Britain’s membership in the European Union.
The decision is a disappointment for May, and there are now questions on whether the British leader will meet that deadline.
FILE - British Prime Minister Theresa May answers a question from the media.
May to appeal decision
At Number 10 Downing Street, the reaction was swift. A spokesman for the prime minister said May still believes she can evoke Article 50 by the end of March. Her government plans to appeal the Supreme Court ruling in December.
"We have no intention of letting this derail our timetable," the spokesman said.
Opponents of Brexit claimed that May’s intention to unilaterally invoke Article 50 was unconstitutional and the process should begin only after receiving input from Parliament.
In principle, the court agreed.
Financial entrepreneur Gina Miller, one of the claimants who challenged plans for Brexit, leaves the High Court in London, Nov. 3, 2016.
In announcing the ruling Thursday, the court said it did not accept the arguments presented by the government, and ruled there is nothing in the law that supports the government’s right to change domestic law on its own, without Parliament’s approval.
"The most fundamental rule of the UK's constitution is that parliament is sovereign," said Lord Chief Justice John Thomas.
Analysts say the court’s decision is not about the principle of Brexit and whether Britain should leave the European Union, but about the specifics of the process by which the voters’ decision will be carried out.
Boost for anti-Brexit politicians
Still, observers say the ruling is a boost for anti-Brexit politicians, who plan to use the court ruling in their arguments in Parliament. In a statement Thursday, Labour party leader Jeremy Corbin praised the ruling, which he said reaffirms there must be “transparency and accountability to Parliament on the terms of Brexit.”
Pro-EU membership supporters argue with a leave campaigner outside the High Court, on the second day of the lawsuit of Gina Miller, a founder of investment management group SCM Private in London, Oct. 17, 2016.
The June referendum was not binding and the British Parliament could, in theory, block Brexit's implementation. A majority of members of Parliament have voiced opposition to Brexit but analysts and polls indicate most would vote in favor of it, since not doing so would amount to political suicide because it would be perceived as going against the will of the people.
The shock vote in June drew 72 percent of Britain’s eligible voters following a bitter campaign that centered on issues of immigration and sovereignty. Polls in the days leading up to the vote had shown a strong lead in favor of those who wanted to remain in the EU.
Proponents of Brexit are now waiting to see what the Supreme Court decides on May's appeal.
"We hope the government would honor the will of the British people to leave the European Union," said Rory Broomfield, director of Better Off Out, a campaign group.
Markets have experienced turbulence since the June vote and the pound has plunged to historic lows, but exports have jumped and economic growth from July through September exceeded forecasts.
The value of the British pound rose after news of Thursday's court ruling.
The months since the vote have been marked by protests among opponents of the decision.