Britain's Prime Minister, Theresa May, leaves Downing Street in London Tuesday Jan. 24, 2017. Britain's government must get parliamentary approval before starting the process of leaving the European Union, the Supreme Court ruled Tuesday, potentially delaying Prime Minister Theresa May's plans to trigger exit negotiations by the end of March. (Victoria Jones/PA via AP)

Britain's Prime Minister, Theresa May, leaves Downing Street in London Tuesday Jan. 24, 2017. Britain's government must get parliamentary approval before starting the process of leaving the European Union, the Supreme Court ruled Tuesday, potentially delaying Prime Minister Theresa May's plans to trigger exit negotiations by the end of March. (Victoria Jones/PA via AP)

LONDON (AP) — Britain’s government warned lawmakers not to try to “thwart the will of the people” after the Supreme Court ruled Tuesday that Prime Minister Theresa May must seek the approval of Parliament before starting the formal process of leaving the European Union.

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 government insisted its timetable of starting the talks by the end of March remained on track, some analysts warned that a defeat in the House of Lords, where May does not have a majority, could delay the process by a year or more.

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“Parliament will rightly scrutinize and debate this legislation,” David Davis, the government’s Brexit secretary, told the House of Commons after the ruling. “But I trust no one will seek to make it a vehicle for attempts to thwart the will of the people, or frustrate or delay the process of our exit from the European Union.”

While the ruling won’t scuttle Britain’s departure, mandated by voters in a June 23 referendum, it once again highlights the uncertainties in negotiating the country’s future relationship with the bloc of 500 million people, which is central to trade, immigration and security. The pound has fallen about 20 percent against the dollar since the vote on concern about slower economic growth and reduced investment.

May’s government fought hard to avoid putting the matter before Parliament, in part because amendments to the legislation could delay her timetable and force her into complicated concessions with her own lawmakers before she even sat across the table before the other 27 members of the EU.

“Unfortunately for businesses and other institutions, Brexit still means uncertainty,” said Phillip Souta, head of UK public policy at the law firm Clifford Chance. “Parliament remains divided and the outcome of the negotiations remain unknown.”

Putting the issue before Parliament is not a simple matter. While May holds sway in the House of Commons, the House of Lords have in recent years stalled dozens of pieces of legislation with which they disagreed, including an attempt by May’s predecessor to impose welfare cuts on the vulnerable. The government backed down after the delay.

“Defeat in the House of Lords would not stop Brexit from happening, but it could delay it until mid-2020,” Souta said.

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 referendum 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 government did win one argument on Tuesday, with the ruling 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. Such consultation would likely have led to a significant delay as lawmakers from the regions piled in with their concerns.

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.

The government of Scotland, where voters overwhelmingly supported continued EU membership, has been an outspoken opponent of the prime minister’s plans for Brexit, and opposition to Tuesday’s ruling was immediately evident. The Scottish National Party, the third-largest party in the House of Commons, pledged to put forward 50 amendments.

The government may be forced to listen: Any sign that its voice is being ignored will feed into the case of those arguing Scotland should be an independent country.

Labour leader Jeremy Corbyn said his party wouldn’t frustrate the process but would seek to amend the legislation to make sure the government is held “accountable.”

Miller, co-founder of SCM Direct, an online investment manager, had argued the case wasn’t about blocking Brexit but 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 meant vindication after months of threats to her security that followed her involvement in the case.

The case revolved around an argument that dates back almost 400 years 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 EU 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.”