The High Court in London will rule Thursday on whether the British government can start the process of leaving the EU without a parliamentary vote, in a landmark constitutional decision that could delay Brexit.

The verdict by three senior judges, expected from 10:00am (1000 GMT), is being closely watched around Europe and by the financial markets as it could derail Prime Minister Theresa May's timetable and affect her negotiating stance.

Most members of the House of Commons campaigned for Britain to stay in the EU in the June referendum, and there is speculation they could push for a softer break with the bloc.

May has said she intends to start formal exit talks by the end of March, and has accused those bringing the court challenge of seeking to frustrate the process, saying: "They're trying to kill it by delaying".

But the claimants -- including an investment fund manager, a hairdresser and an expatriate living in France -- argue that Britain was taken into the EU by parliament, and only parliament can made the decision to leave.

"Judgement Day," tweeted expat Grahame Pigney, who has raised funds through crowdsourcing for his part in the case.

During three days of hearings in October, May conceded that parliament would likely have a vote on the final deal negotiated with the bloc.

But the case is challenging her right to use "historic prerogative powers" -- a type of executive privilege -- to trigger notification of Article 50 of the EU's Lisbon Treaty, setting off a two-year countdown to Brexit.

"This is not about whether we should stay or leave -- this is actually about how we leave," Gina Miller, the co-founder of investment fund SCM Private and the lead claimant in the case, told AFP last month.

She added: "If we bypass or we set a precedent... that a prime minister can decide what rights we have and what rights we don't, then basically we go back to being a dictatorship and we roll back democracy 400 years."

Whoever loses the case on Thursday is likely to appeal, and because of the time constraints, the issue is likely to go straight to the Supreme Court for final deliberation before the end of the year.

The case is being heard by England's two top judges -- Lord Chief Justice John Thomas and Master of the Rolls Terence Etherton -- and Philip Sales, an appeal court judge.

Source: AFP