The High Court ruled Thursday that the British government cannot start the process of leaving the EU without the approval of parliament, in a landmark judgement 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 judgement."
Most members of parliament's lower 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 intends to trigger Article 50 between the New Year and 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 maximum 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.
May previously accused those behind the legal challenge of seeking to frustrate the Brexit process, saying: "They're trying to kill it by delaying it."
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 make the decision to leave.
"I am grateful to the court for the result. This is a victory for parliamentary democracy," said hairdresser Deir Dos Santos, in a statement read outside court by his lawyer David Greene.
Dos Santos -- who voted for Brexit -- condemned the prime minister for her "unwarranted and irresponsible attack" on the case.
"I now hope everyone will respect the court's decision," he said.
But Nigel Farage, the interim leader of the UK Independence Party who campaigned vocally for Brexit, warned there would be public outrage if the referendum result was not implemented.
"I worry that a betrayal may be near at hand," he said.
"I now fear that every attempt will be made to block or delay the triggering of Article 50. If this is so, they have no idea of the level of public anger they will provoke."
During three days of hearings in October, May conceded that parliament would likely have a vote on the final deal negotiated with the bloc.
The case was 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.