The UK Supreme Court ruled on Tuesday that Prime Minister Theresa May must get parliament's approval before she begins Britain's formal exit from the European Union.

The UK's highest judicial body dismissed the government's argument that May could simply use executive powers known as "royal prerogative" to invoke Article 50 of the EU's Lisbon Treaty and begin two years of divorce talks. However, the court rejected arguments that the UK's devolved assemblies in Northern Ireland, Scotland and Wales should give their assent before Article 50 is invoked.

"The referendum is of great political significance, but the Act of Parliament which established it did not say what should happen as a result," said David Neuberger, President of the Supreme Court which ruled by 8-3 against the government. "So any change in the law to give effect to the referendum must be made in the only way permitted by the UK constitution, namely by an Act of Parliament."

Britain's Attorney General Jeremy Wright said the government would implement a UK Supreme Court decision that Prime Minister Theresa May must obtain parliament's approval before she begins Britain's formal exit from the European Union.

"Of course the government is disappointed with the outcome," Wright said outside the Supreme Court. "The government will comply with the judgment of the court and do all that is necessary to implement it," he said.

May has repeatedly said she would trigger Article 50 before the end of March but she will now have to seek the consent of lawmakers first, potentially meaning her plans could be amended or delayed, although the main opposition Labor Party has said it would not slow her timetable.

Last week May set out her stall for negotiations, promising a clean break with the world's largest trading block as part of a 12-point plan to focus on global free trade deals, setting out a course for a so-called "hard Brexit".

The greatest potential threat to May comes from parliament's unelected upper chamber, the House of Lords, where many peers remained strongly opposed to Brexit and do not have voters to worry about.

If the Lords were to vote against approving the triggering of Article 50, the Brexit timetable could be severely delayed.

However, the government is confident the bill will pass through the Lords because there would be a constitutional crisis if unelected peers were to thwart the will of the people expressed both through the referendum and from their representatives in the Commons.