London: Britain's prime minister cannot trigger Brexit single-handedly, the country's High Court has ruled, in a decision that embarrassed the government and gave new hope to "Remainers".
After a high-stakes argument over the balance of power between Parliament and the Crown, the court ruled unanimously that the "royal prerogative" did not give the government power to start the legal process of taking Britain out of the European Union.
Prime Minister Theresa May had planned to trigger the "Article 50" treaty exit clause by the end of March, but must now first seek the approval of parliament.
This will open the door to Remainers who wish to dilute or even block Brexit.
Before the Brexit referendum a majority of MPs were against leaving the EU. Now many, including the prime minister, take the view that it would be undemocratic – or electoral suicide – to go against the expressed will of the people.
But MPs may seek assurances that Britain will not negotiate for a "hard" Brexit, instead pushing to stay in the single market and the customs union, lessening the predicted damage to the economy.
Liberal Democrat leader Tim Farron said the decision gave MPs "the chance to say no to an irresponsible hard Brexit that risks our economy and our jobs".
The House of Lords, which does not have to answer to voters at future elections, could prove an even more challenging hurdle to jump than the Commons.
UKIP interim leader Nigel Farage warned Britain was heading for a "half-Brexit" and the ruling was "the beginning of a process where there is a deliberate, willful attempt by our political class to betray 17.4 million voters".
Former Lib Dem leader Menzies Campbell said it was a "slap in the face" for May.
The decision is doubly embarrassing for the government because the Crown's legal team was led by the attorney-general himself.
According to a reporter for The Times newspaper in Britain, the prime minister "believes that government lawyers haven't covered themselves in glory" over their handling of the case.
They will get another try, as the government will appeal the decision to the Supreme Court. A government spokesman said: "The country voted to leave the European Union in a referendum approved by Act of Parliament, and the government is determined to respect the result of the referendum. We will appeal this judgment."
The prime minister reportedly believes they can stick to the March timetable.
The court's ruling was welcomed on both sides of parliament.
"It's all about parliament's sovereignty," Conservative MP Anna Soubry said. "It's this place which will and indeed should trigger Article 50."
Barry Gardiner, shadow international trade secretary, said the Brexit deal "should not be smuggled through in secret" but subject to parliamentary scrutiny.
Trade Secretary Liam Fox said the government would have to wait for the result of the appeal before it settled on a plan of action.
"The most fundamental rule of UK constitutional law is that the Crown in Parliament is sovereign," the judges said in their 32-page decision. "The powerful constitutional principle that the Crown has no power to alter the law of the land by use of its prerogative powers is the product of an especially strong constitutional tradition in the United Kingdom… it evolved through the long struggle to assert parliamentary sovereignty and constrain the Crown's prerogative powers."
Lawyers opposing the government had cited centuries-old cases, dating from a time when King James I had tried – and failed – to make proclamations that overruled parliament.
The government still has royal prerogative powers, such as signing international treaties, but old precedents established they cannot overrule Parliament by changing domestic law, and they cannot deprive British citizens of their rights.
"Withdrawal from the European Union pursuant to Article 50 would undo … rights of major importance created by Parliament," the court found. "It would be surprising if they could be removed simply through action by the Crown."
Pro-Brexit politicians had also argued – though not before the court – that parliament had anticipated the consequences of a "yes" vote when it passed the law setting up the Brexit referendum.
However, the High Court ruled this was an "untenable" argument, because "a referendum on any topic can only be advisory for the lawmakers in Parliament unless very clear language to the contrary is used".
It concluded "the Secretary of State does not have power under the Crown's prerogative to give notice… for the United Kingdom to withdraw from the European Union".
The decision was welcomed by currency markets with a sharp rise in the value of the pound against the dollar, though it is a long way short of its pre-referendum value.