In a huge setback for British Prime Minister Theresa May that could delay her Brexit plans, the Supreme Court on Tuesday ruled that she cannot trigger the process of taking Britain out of the European Union (EU) unilaterally and must seek Parliament's consent over it.
The judgement means the British Prime Minister cannot trigger Article 50 of the Lisbon Treaty to officially begin Brexit negotiations with the EU until Britain s MPs and peers give their backing.
The government had argued that it already had the executive powers to trigger Article 50 but Supreme Court judges rejected the case by a majority of eight to three.
Reading out the ruling, Supreme Court President Lord Neuberger said: "By a majority of eight to three, the Supreme Court today rules that the Government cannot trigger Article 50 without an Act of Parliament authorising it to do so."
The official judgement in the case concludes: "The change in the law required to implement the referendum s outcome must be made in the only way permitted by the UK constitution, namely by legislation."
In a joint judgment of the majority, the Supreme Court holds that an Act of Parliament is required to authorise ministers to give notice of the decision of the UK to withdraw from the European Union."
The government had lost a High Court case brought by anti-Brexit campaigners last November, leading to the landmark appeal in the Supreme Court heard by all 11 justices of Britain s highest court.
The hearing also included arguments from Scotland, Wales and Northern Ireland as interested parties in the case.
The Supreme Court, however, ruled that the devolved Scottish Parliament and Welsh and Northern Ireland assemblies did not need a say in the formal Brexit process.
Gina Miller, the lead campaigner in the case, said: "Brexit is the biggest decision for a generation. But this case was about process, not politics.
"Only Parliament can grant right to the British people and only parliament can take them away. No prime minister, no government, can expect to be unanswerable or unchallenged.
Parliament alone is sovereign."
UK Attorney General Jeremy Wright said the government was "disappointed" but would "comply" and do "all that is necessary" to implement the court's judgement.
Downing Street stressed that the ruling will not derail May's timetable to trigger Article 50 by the end of March.
"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," a Downing Street spokesperson said.
"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. We respect the Supreme Court's decision, and will set out our next steps to Parliament shortly," the spokesperson added.
UK Secretary of State for Exiting the European Union David Davis is expected to make a statement in the House of Commons.
Downing Street has been preparing for the result for weeks and is understood to have drafted a short bill to seek parliamentary approval to trigger Article 50.