British Prime Minister Theresa May must give Parliament a vote before she can formally start Brexit, the UK Supreme Court has ruled, giving politicians who oppose her plans a shot at amending them.

Key points: Supreme Court voted against Government 8–3

Supreme Court voted against Government 8–3 Triggering Brexit now only possible by an "Act of Parliament"

Triggering Brexit now only possible by an "Act of Parliament" Court also decided no need for Scotland, Wales or Northern Island to give assent before Brexit

By a majority of eight-to-three, the highest judicial body in the UK dismissed the Government's argument that Ms May could simply use executive powers known as "royal prerogative" to invoke Article 50 of the EU's Lisbon Treaty.

"The referendum is of great political significance, but the Act of Parliament which established it did not say what should happen as a result," Supreme Court president David Neuberger said.

"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."

However, the judges did remove one major potential obstacle for the British Government, saying Ms May did not need the approval of the UK's devolved assemblies in Scotland, Wales and Northern Ireland before triggering Brexit.

Ruling 'does nothing' to change March timeline

Ms May has repeatedly said she would trigger Article 50 before the end of March, but the ruling means she must now first bring forward urgent legislation before she can go ahead.

That opens up the Brexit process to scrutiny from politicians, the majority of whom had wanted to stay in the EU.

However, a spokesperson for Ms May said the High Court ruling did "nothing to change" the planned March deadline.

"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," the spokesman said.

"We respect the Supreme Court's decision, and will set out our next steps to Parliament shortly."

The Labour Party said it would not slow down Ms May's timetable, but it would be looking to "build in" maintenance of workers' rights and tariff-free access to the single market in the bill.

"Labour will seek to amend the Article 50 bill to prevent the Conservatives using Brexit to turn Britain into a bargain basement tax haven off the coast of Europe," a spokesperson for Labour Party leader Jeremy Corbyn said.

Meanwhile, the Scottish National Party, which has 54 MPs, vowed to put forward 50 "serious and substantive" amendments.

However, the opponents of Brexit are still likely to be some way short of the numbers needed to either delay Ms May's timetable or to stop it.

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

Investors 'can focus on other things' after ruling

Some investors and those who backed the Remain campaign hope that politicians, most of whom wanted to stay in the EU, will force Ms May to seek a deal which prioritises access to the European single market of 500 million people, or potentially even block Brexit altogether.

The British Sterling initially rose on the news that the Government had lost its appeal, but it then fell over half a cent against the dollar and the euro after the court ruled that Scotland, Wales and Northern Ireland did not need to give their assent to triggering Article 50.

Sterling last traded down 0.5 per cent on the day at $US1.2473 ($1.65).

"The court ruling is a slap in the face of the British Government," said Jawaid Afsar, senior trader at Securequity.

"However, Parliament is likely to give its approval and the Brexit timeline could remain on track. As far as investors are concerned, one more uncertainty is now out of the way and they can focus on other things."

AP/Reuters