LONDON (AFP) -- The High Court in London ruled Thursday that the British government alone cannot start the process of leaving the European Union but requires the approval of parliament, in a landmark judgment that could delay Brexit.

Three senior judges said Prime Minister Theresa May's government does not have the power itself to trigger Article 50 of the EU's Lisbon treaty, the formal notification of Britain's intention to leave the bloc.

"We hold that the Secretary of State does not have power under the Crown's prerogative to give notice pursuant to Article 50... for the United Kingdom to withdraw from the European Union," the judgement said.

May's Downing Street office said it was "disappointed" at the decision and would appeal, with the case now expected to be heard in the Supreme Court in early December.

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

"And the government is determined to respect the result of the referendum. We will appeal this judgment."

Most members of the House of Commons wanted Britain to stay in the EU in the June referendum, and there is speculation they could push for a softer break with the bloc or even try to prevent it altogether.

The pound rallied against the dollar and euro after the High Court ruling, jumping above $1.24 after weeks of tumbling to multi-year low points against its main rivals.

May announced last month that she intends to trigger Article 50 by the end of March, a move welcomed by EU leaders who are pressing for a swift divorce to limit uncertainty over the future of Britain and the rest of the bloc.

But the timetable may be derailed by the case, which challenged her right to use "historic prerogative powers" -- a type of executive privilege -- to make that decision. 

Article 50 notification begins a two-year countdown to withdrawal and lawmakers are now likely to demand more information -- and more of a say -- on the government's negotiating strategy before giving their approval.