The High Court in London has ruled that parliament, not the government, must approve the start of Britain's withdrawal from the European Union, in a landmark decision that could delay Brexit.

It is one of the most constitutionally significant and politically volatile issues in a generation.

The verdict by three senior judges was closely watched around Europe and by the financial markets as it could derail British Prime Minister Theresa May's plans and affect her negotiating strategy.

Following the June referendum vote for Britain to leave the European Union, Ms May had promised to start formal exit talks by the end of March.

She claimed she had the right to use "historic prerogative powers" - a type of executive privilege - to trigger notification of Article 50 of the EU's Lisbon Treaty, which would spark two years of negotiations on Britain's departure from the bloc.

"The court does not accept the argument put forward by the government," the judges said in a statement read out to the court.

"For the reasons set out in the judgment, we decide that the government does not have power ... to give notice pursuant to Article 50 for the UK to withdraw from the European Union."

The Lord Chief Justice declared that the most fundamental rule of the UK constitution is that parliament is sovereign.

Two-thirds of the House of Commons backed the Remain side in the referendum.

It is not believed that parliament would vote against Brexit and the clear choice of the British electorate to leave the EU.

It is thought a move through the House of Commons would be more likely to result in a soft Brexit and a delay in the timetable set out by the prime minister.

What is next in the Brexit process?

Those behind the legal challenge - including an investment fund manager, a hairdresser and an expatriate living in France - had argued that Article 50 could not be triggered without a decision by parliament.

They claimed leaving the EU without such a move would remove statutory rights enshrined under the European Communities Act 1972, which made EU law part of UK law.

The campaigner who spearheaded the legal battle said she was absolutely delighted with the ruling.

Speaking outside the court, Gina Miller said: "The result today is about all of us, it's not about me or my team.

"It's about our United Kingdom and all out futures. It's not about how anyone voted.

"Every one of us voted for the best country and the best future."

She added: "This case was about process not politics."

A statement from the prime minister's office said: "We will appeal this judgment."

"The country voted to leave the European Union in a referendum approved by Act of Parliament," it added.

A government lawyer said the Supreme Court had set aside 5-8 December to hear the matter.

European Commission President Jean-Claude Juncker is to speak by telephone to Ms May tomorrow morning at the British prime minister's request, a spokesman for the commission said.

Meanwhile, Sinn Féin president Gerry Adams has ruled out changing the party's abstentions policy at Westminster, even on a temporary basis, to vote on Brexit if the issue comes before the parliament.

Asked about it at Stormont, Mr Adams said: "No and you knew the answer to that before you asked me."

The Sinn Féin attitude has been criticised by SDLP leader, Colum Eastwood who said the party's three Westminster MP's would be voting 'No' to Brexit and reflecting the views of a majority of voters in Northern Ireland if given a chance to do so.

Last week, Northern Ireland's High Court rejected a legal challenge to Brexit, the first judicial ruling on the issue in the UK.

The challenge rested on assumptions spiralling from the Good Friday Agreement peace accords that Northern Ireland's constitutional arrangements could only be changed with the consent of its residents.

Sterling jumps as government loses Brexit ruling