Jane Onyanga-Omara, and Kim Hjelmgaard

USA TODAY

LONDON — Britain's top court ruled Tuesday that Prime Minister Theresa May's government must seek parliamentary approval before it can start the formal process to withdraw from the European Union.

While the judgment is a political blow to the British leader and will complicate the route to Brexit, experts say it is unlikely to have a major impact on the process.

The Supreme Court's 8-3 decision forces May to have Parliament vote before she can trigger Article 50 of the EU's treaty, the legislation that allows the United Kingdom to start negotiating its exit from the 28-nation alliance. May had vowed to enact Article 50 by the end of March.

“The U.K’s constitutional arrangements require such changes to be clearly authorized by Parliament," said Lord Neuberger, president of the Supreme Court.

“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. To proceed otherwise would be a breach of settled constitutional principles stretching back many centuries," he added.

Tuesday's ruling means that May is not able to begin EU exit talks until she gets consent from British legislators. Lawmakers from the major parties have said they want to abide by the results of the contentious June referendum that saw a majority of Britons vote to leave the EU.

The legal case against the government was brought by a hairdresser, a plumber and an investment banker. The court also ruled — unanimously — that regional British legislatures in Northern Ireland, Scotland and Wales are not entitled to have a say in how and when Article 50 is triggered.

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After the ruling, May's office said in a statement that it would trigger Article 50 by the end of March, as planned. "Today’s ruling does nothing to change that," the statement said.

Brexit Secretary David Davis told lawmakers that the government will introduce legislation "within days" allowing Parliament to move to invoke Article 50. "This will be a straightforward bill," he said.

Scottish leader Nicola Sturgeon called the ruling a "damning indictment of a U.K. government that believed it could press on toward a hard Brexit with no regard to Parliament whatsoever."

A "hard" Brexit, which May has promised, means the U.K. would leave the single European market, which allows EU member nations to trade with each other without restrictions. Sturgeon has warned that Scotland would hold a second referendum calling for independence if the British government rejects her plan to keep Scotland in the single market after Britain leaves the EU.

Sturgeon said lawmakers from her Scottish National Party will seek to work with other British members of Parliament "to stop the march toward a hard Brexit in its tracks." She said that while the U.K. government isn't legally required to consult the regional legislatures, there was still "a clear political obligation" to do so.

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Jeremy Corbyn, leader of the opposition Labour Party, said his party respects the referendum result and "will not frustrate the process for invoking Article 50."

May had argued to the court that she did not need parliamentary consent to implement Article 50 because of executive authority granted to the government under the "royal prerogative" — discretionary powers sometimes used to opt out of international treaties. Opponents argued that parliamentary approval is needed to ward off a constitutional crisis.

The British leader said last week she would have Parliament vote on the Brexit deal once it's negotiated with the EU, a process expected to take two years.

Matthew Goodwin, a professor of politics and international relations at the University of Kent, said the court ruling will embolden those against Brexit who wanted to block the move, but it will also anger those who voted to leave the EU, making them "likely to feel as though the will of the people is being thwarted by elites in London."

"Nonetheless, given that a large majority of members of Parliament will vote to trigger Article 50, the ruling is, overall, unlikely to have a major impact on the Brexit process,” Goodwin said.

Gina Miller, the businesswoman and philanthropist who was the lead person to bring the lawsuit, said outside the court: "No prime minister, no government, can expect to be unanswerable or unchallenged. This ruling today means that MPs we have elected will rightfully have the opportunity to bring their invaluable experience and expertise ... in the forthcoming Brexit negotiations."

"There is no doubt that Brexit is the most divisive issue of a generation," she said, "but this case was about the legal process, not politics."

Hjelmgaard reported from Berlin.