LONDON — (CNN)  Britain’s Supreme Court has ruled that the UK government must hold a vote in parliament before beginning the process of leaving the European Union.

The decision is a complication for Prime Minister Theresa May, who wanted to trigger Article 50 of the Lisbon Treaty — the legal mechanism that begins the process of leaving the EU — by the end of March. Doing so would open the door for EU negotiations, which are likely to last two years.

May had promised lawmakers a vote on the outcome of the talks, but wanted to begin the process without a decision in parliament.

What the court said

The Supreme Court judges voted eight to three against the government, upholding a November High Court decision. The judges, who deliberated the case over four days in December, said that the legal consequences of leaving the EU were great enough to require an act of parliament to start the process.

“To proceed otherwise would be breach of settled constitutional principles stretching back many centuries,” Lord David Neuberger, president of the Supreme Court, said as he read out the ruling.

But the court decided that the UK government did not need the approval of devolved governments in Scotland, Northern Ireland and Wales to begin the negotiating process.

That decision is likely to be met with relief at 10 Downing Street, as 62% of Scots voted to remain in the union, and Scotland’s First Minister Nicola Sturgeon has remained opposed to the country’s EU withdrawal.

What the UK government said

The government said it was disappointed by the ruling, but would abide by it.

Jeremy Wright, the Attorney General for England and Wales, told reporters outside the court that ministers would “comply with the judgment of the court and do everything necessary to implement it.”

But a spokesperson for Theresa May said the ruling would not change the government’s Brexit timeline.

“The British people voted to leave the EU, and the government will deliver on their verdict — triggering Article 50, as planned, by the end of March. Today’s ruling does nothing to change that,” the spokesperson said in a statement.

“It’s important to remember that parliament backed the referendum by a margin of six to one and has already indicated its support for getting on with the process of exit to the timetable we have set out.”

Brexit has become a key test of Prime Minister May’s leadership. May took over the premiership after former leader David Cameron stood down over the Brexit vote. Cameron had called the referendum but had campaigned for the country to remain in the union.

What happens next

The government must now introduce legislation to the House of Commons, which could delay May’s plan to trigger article 50 by the end of March.

The opposition Liberal Democrat Party and Scottish National Party (SNP) are opposed to Brexit, and are likely to put up the hardest fight. The SNP said on Tuesday that it would table 50 “serious and substantive” amendments to the government’s bill.

Tim Farron, the leader of the Liberal Democrats, said the party would vote against the triggering of Article 50 unless the government promised a referendum on the terms of the final exit deal with the EU.

But the Labour party, the largest opposition group in the House of Commons, has already said it would not stand in May’s way. That means the government is likely to win the eventual vote on Article 50.

Reaction to the ruling

One of the claimants in the case, London-based businesswoman Gina Miller, welcomed the decision, saying Brexit had been “most divisive issue of a generation.”

“Only parliament can grant rights to the British people and only parliament can take them away. No prime minister and no government can expect to be unanswerable or unchallenged. Parliament alone is sovereign,” Miller said after the ruling.

She told of how she had been harassed over the course of the case. CNN previously reported that she had received rape and death threats over the issue.

She said she was “shocked by the levels of personal abuse that I have received from many quarter over the last seven months for simply bringing and asking a legitimate question.”

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