The UK parliament alone has the power to trigger Brexit by notifying Brussels of the UK's intention to leave the European Union, the high court has ruled.
The judgment, delivered by the Lord Chief Justice, Lord Thomas of Cwmgiedd, is likely to slow the pace of Britain's departure from the EU and is a huge setback for Theresa May, who had insisted the government alone would decide when to trigger the process.
The lord chief justice said that ''the most fundamental rule of the UK constitution is that parliament is sovereign''.
A government spokesman said ministers would appeal to the Supreme Court against the decision. The hearing will take place on 7-8 December.
Lord Thomas said, ''The court does not accept the argument put forward by the government. There is nothing in the 1972 European Communities Act to support it. In the judgment of the court, the argument is contrary both to the language used by parliament in the 1972 Act, and to the fundamental principles of the sovereignty of parliament and the absence of any entitlement on the part of the crown to change domestic law by the exercise of its prerogative powers.''
Unless overturned on appeal at the Supreme Court, the ruling threatens to plunge the government's plans for Brexit into disarray as the process will have to be subject to full parliamentary control.
Government lawyers had argued that prerogative powers were a legitimate way to give effect ''to the will of the people'' who voted by a clear majority to leave the EU in the June referendum.
But Lord Thomas declared, ''The government does not have power under the crown's prerogative to give notice pursuant to Article 50 for the UK to withdraw from the European Union.''
The international trade secretary, Liam Fox, said the government was disappointed by the high court decision but added that ''the government is determined to respect the result of the referendum''.
The Ukip leader, Nigel Farage, said he was angered by the decision. ''I worry that a betrayal may be near at hand … 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.''
The Labour leader, Jeremy Corbyn, said, ''This ruling underlines the need for the government to bring its negotiating terms to parliament without delay. Labour respects the decision of the British people to leave the European Union. But there must be transparency and accountability to parliament on the terms of Brexit.''
The Liberal Democrat leader, Tim Farron, said he was delighted by the ruling. ''Given the strict two-year timetable of exiting the EU once Article 50 is triggered, it is critical that the government now lay out their negotiating to parliament, before such a vote is held,'' he said.
By handing responsibility for initiating Brexit over to MPs, the three senior judges – Lord Thomas, the master of the rolls, Sir Terence Etherton, and Lord Justice Sales – have ventured on to constitutionally untested ground.
The legal dispute focused on Article 50 of the treaty on European Union, which says any member state may leave ''in accordance with its own constitutional requirements'' – an undefined term that has allowed both sides to pursue rival interpretations.
The decision may undermine the prime minister's authority in conducting negotiations with other EU states in the run-up to the UK's withdrawal.
Gina Miller, the lead claimant in the case, said, ''It was the right decision because we were dealing with the sovereignty of parliament. It was not about winning or losing. It was about what was right. Now we can move forward with legal certainty.''
Deir Dos Santos, a hairdresser and the other lead claimant, said, ''Today's judgment is a victory for everyone who believes in the supremacy of our parliament and the rule of law. I have never challenged the result of the referendum – in fact I voted for Brexit for the sole reason that I wanted power to be returned from Europe to the British parliament. But I did not think it was right for the government then just to bypass parliament and try to take away my legal rights without consulting parliament first.''
John Halford, the solicitor at Bindmans who represented the People's Challenge group, said, ''The oversight, control and democratic accountability needed for decisions on Brexit have to match the consequences of those decisions for UK citizens. That is why our constitution empowers parliament, not the government, to take decisions.''
John Shaw, chair of the organisation Fair Deal for Expats, said, ''This is superb news. We were convinced that our case was just. We're delighted that the court agrees with us. There now needs to be a proper debate about how the rights of expats will be protected.''
The three judges' ruling was unanimous. It stated, ''By making and unmaking treaties the crown [ie the government] creates legal effects on the plane of international law, but in doing so it does not and cannot change domestic law. It cannot without the intervention of parliament confer rights on individuals or deprive individuals of rights.''
At one point the judgment dismisses arguments deployed by lawyers for the government – about whether rights within the EU were conferred by act of parliament or international treaty – as being ''divorced from reality''.
The judges said, ''The reality is that parliament knew and intended that enactment of the European Communities Act 1972 would provide the foundation acquisition by British citizens of rights under EU law which they could enforce in other member states.''
They added that ''the claimants are entitled to say that it would be surprising if they could be removed simply through action by the crown under its prerogative powers''.
They concluded, ''In our judgment, the clear and necessary implication from these provisions taken separately and cumulatively is that parliament intended EU rights to have effect in domestic law and that this effect should not be capable of being undone or overridden by action taken by the crown in exercise of its prerogative powers.''
Part of the judges' reasoning was based on legal precedents dating back to the 13th century, in particular the Case of Proclamations. That case involved merchants who were prevented from working in London by proclamation of Henry IV which was found to be in breach of a parliamentary act dating back to 1297. Parliament triumphed and the crown had to withdraw its ban.