LONDON (AP) — Britain’s Supreme Court ruled Tuesday that Prime Minister Theresa May must get legislative approval to start the process of leaving the European Union, raising the possibility that lawmakers will delay her plans to trigger negotiations by the end of March.
The 8-3 decision forces the government to put a bill before Parliament, giving members of the House of Commons and the unelected House of Lords the chance to debate and potentially offer amendments that could soften the terms of Britain’s exit from the EU, known as Brexit.
While the ruling won’t scuttle Britain’s departure, it once again highlights uncertainty about the timetable for negotiating the country’s future relationship with the bloc of 500 million people, which is central to trade, immigration and security.
“Unfortunately for businesses and other institutions, Brexit still means uncertainty,” said Phillip Souta, head of U.K. public policy at the international law firm Clifford Chance. “Parliament remains divided and the outcome of the negotiations remain unknown.”
The lawsuit was considered the most important constitutional case in a generation because it centered on the question of who ultimately wields power in Britain’s system of government: the prime minister and her Cabinet, or Parliament.
May had said she would use centuries-old powers known as royal prerogative to invoke Article 50 of the EU treaty and launch two years of exit talks. The powers — traditionally held by the monarch — permit decisions about treaties and other specific issues to be made without a vote of Parliament.
The prime minister argued that the June 23 referendum on EU membership gave her a mandate to take Britain out of the 28-nation bloc and that discussing the details of her strategy with Parliament would weaken the government’s negotiating position.
Financial entrepreneur Gina Miller sued to force the government to seek parliamentary approval. Leaving the EU, she said, would change the fundamental rights of citizens and this can’t be done without a vote of lawmakers. The Supreme Court agreed.
“The referendum is of great political significance, but the act of Parliament which established it did not say what should happen as a result, so any change in the law to give effect to the referendum must be made in the only way permitted by the U.K. constitution, namely by an act of Parliament,” Supreme Court President David Neuberger said in reading the decision.
“To proceed otherwise would be a breach of settled constitutional principles stretching back many centuries,” he said.
Significantly, the court also ruled that the legislatures of the nations that are a part of the United Kingdom — Scotland, Wales and Northern Ireland — do not need to be consulted on Brexit.
While the U.K. government has ceded authority over many local issues to these bodies, responsibility for international relations still rests with the government in Westminster.
“Relations with the EU are a matter for the UK government,” Neuberger said.
A decision in favor of the regional governments would likely have led to even more delays as local lawmakers piled in with their concerns. The government of Scotland, where voters overwhelmingly supported continued EU membership, has been an outspoken opponent of the prime minister’s plans for Brexit.
The court’s ruling will focus the debate in Parliament, where May’s Brexit secretary, David Davis, told the House of Commons that the government timetable remained on track. A bill will be introduced within days.
“There can be no turning back,” Davis said. “The point of no return was passed on June 23 last year.”
Opposition was immediately evident.
Labour leader Jeremy Corbyn said his party would seek to amend the legislation to make sure the government is held “accountable.” The Scottish National Party, the third-largest party in the House of Commons, promised to put forward 50 amendments.
Miller, co-founder of SCM Direct, an online investment manager, had argued the case wasn’t about blocking Brexit. Instead, she said, it was about “democracy” and the “dangerous precedent” that a government can overrule Parliament.
For Miller, who brought the case along with hairdresser Deir Dos Santos, the court’s decision brought vindication after months of threats to her security that followed her involvement in the case.
“No prime minister, no government can expect to be unanswerable or unchallenged,” she said. “Parliament alone is sovereign.”
The case revolved around an argument that dates back almost 400 years to the English Civil War as to whether power ultimately rests in the executive or Parliament.
Constitutional expert Andrew Blick, an expert on the Magna Carta at King’s College London, said that advocates of withdrawal had long argued that leaving the 28-nation bloc would protect parliamentary sovereignty.
“They claimed that leaving would promote the principle that the U.K. Parliament is the ultimate source of constitutional authority in the UK,” said Blick, who advised the Welsh government on the case. “That principle has now come back to bite them.”
Underscoring the importance of the case, May put Attorney General Jeremy Wright in charge of the legal team fighting the suit. Wright had argued the suit was an attempt to put a legal obstacle in the way of enacting the referendum result.
The decision is a bad defeat for May and means that the government “still does not have control of the Brexit timetable,” said David Allen Green, a lawyer at the London legal firm Preiskel & Co.
“The appeal decision is, however, a victory for the doctrine of parliamentary sovereignty and a vindication of an independent judiciary,” Green said. “The Supreme Court has told the government to get back into its box: A proper process has to be followed.”