The Supreme Court has today ruled that the UK government cannot trigger Article 50 and enact Brexit without an Act of Parliament. 

Ministers will now have to ask Parliament to vote on legislation invoking Article 50 of the Lisbon Treaty, thus formally notifying the rest of the EU of the UK’s decision to leave.

The landmark judgement doesn’t overturn the result of the referendum. Instead, it is a ruling on how Article 50, the EU’s exit clause, will be triggered – defining who exactly has the legal power to change the rights of British citizens.

It represents a defeat for Theresa May’s government and an upholding of a previous High Court judgement, which said that MPs and peers opposed to Brexit should get the chance to vote against triggering Article 50 in Parliament.

Ministers will now have to ask all Members of Parliament – including pro-EU MPs and peers – to vote on legislation triggering Article 50.

If the government had won its appeal, the Prime Minister could go straight to Brussels to formally notify the rest of the EU of the UK’s decision to leave without first having to secure approval from the rest of Parliament.

Theresa May’s government had argued that ministers had the power to make the decision using “prerogative powers” or the Royal Prerogative, a remnant of the era of all-powerful kings and queens.

However, the government’s opponents successfully argued that ministers needed the approval of Parliament before starting the two years of negotiations.

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May’s government is now expected to swiftly publish legislation asking Parliament to invoke Article 50, in the hope that it will be approved by MPs and peers by March.

The case against the government was brought by Gina Miller, an investment fund manager and philanthropist. Miller launched the Brexit legal case with London-based Spanish hairdresser Deir Dos Santos and the People’s Challenge Group.

Miller argued that only Parliament could make a decision leading to the loss of her “rights” under EU law. However, she emphasised that her case was not an attempt to block the referendum decision.

Speaking outside the Supreme Court on Tuesday, Miller said that the case “went to the very heart of our constitution and how we are governed.”

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“Only parliament can grant right to the British people and only parliament can take them away,” she said. “No prime minister, no government, can expect to be unanswerable or unchallenged.”

She added: “There is no doubt that Brexit is the most divisive issue of a generation. But this case was about the legal process, not politics.”

Miller also said she had been “shocked by the levels of personal abuse that I have received from many quarters over the last seven months, for simply bringing and asking a legitimate question.”

The Labour leader, Jeremy Corbyn, said that his MPs would not “frustrate the process for invoking Article 50” by voting against it in Parliament.

Instead, he said Labour would seek to amend the bill by building in “full, tariff-free access to the single market and maintenance of workers’ rights and social and environmental protections.”

However, several pro-EU members of Corbyn’s party, including up to half of London’s Labour MPs, are said to be considering rebelling against Corbyn and voting against invoking Article 50 in March.

Tulip Siddiqi, the Labour MP for Hampstead and Kilburn, said that she would consider resigning if Corbyn continued to support Brexit.

“Three quarters of my constituents voted to remain and I will stand up for them,” she said.

The leader of the Liberal Democrats, Tim Farron, said that his party would vote against Article 50 unless the British people were given another vote on the final deal.

Following the Supreme Court ruling, a spokesperson for Number 10 said: “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.”

Images: Rex Features