Parliament must vote on whether the UK can start the process of leaving the EU, the High Court has ruled.
This means the government cannot trigger Article 50 of the Lisbon Treaty – beginning formal discussions with the EU – on their own.
Theresa May says the referendum – and existing ministerial powers – mean MPs do not need to vote, but campaigners called this unconstitutional.
The government is appealing, with a further hearing expected next month.
Labour leader Jeremy Corbyn urged the government “to bring its negotiating terms to parliament without delay”, adding that “there must be transparency and accountability to Parliament on the terms of Brexit”.
UKIP leader Nigel Farage said he feared a “betrayal” of the 51.9% of voters who backed leaving the EU in June’s referendum.
BBC assistant political editor Norman Smith said, if the court’s decision was not overturned, there could be delays with potentially “months and months” of parliamentary hurdles.
But there was not yet “clarity” – if the judgement was not overturned – on whether there would be a “short, sharp” vote or whether Parliament would have to consider complex legislation, he added.
He predicted that most MPs would ultimately be likely to vote for Article 50, as Brexit had been backed by a majority of voters in the referendum.
The prime minister has said she will activate Article 50, formally notifying the EU of the UK’s intention to leave, by the end of next March.
The other 27 member states have said negotiations about the terms of the UK’s exit – due to last two years – cannot begin until Article 50 has been invoked.
Gina Miller, who brought the case, said outside the High Court that the government should make the “wise decision of not appealing”.
She said: “The result today is about all of us. It’s not about me or my team. It’s about our United Kingdom and all our futures.”
But a government spokesman announced it would contest the ruling at the Supreme Court.
He 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 judgement.”
‘Give British people a chance to say no’
Government lawyers had argued that prerogative powers were a legitimate way to give effect “to the will of the people”.
But the Lord Chief Justice 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.”
Reacting to the ruling, International Trade Secretary Liam Fox told the House of Commons the government was “disappointed” but remained “determined to respect the result of the referendum”.
He added: “There will be numerous opportunities for the House to examine and discuss what the Government is negotiating.
“When we are clear about the position we will adopt, then Article 50 will be triggered but, given the nature of the judgement this morning, we will now have to await the government’s appeal to the Supreme Court.”
Mr Farage said: “I worry that a betrayal may be near at hand.”
He added: “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.”
Mr 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.”
But Liberal Democrat leader Tim Farron, a supporter of remaining in the EU, said: “Ultimately, the British people voted for a departure but not for a destination, which is why what really matters is allowing them to vote again on the final deal, giving them the chance to say no to an irresponsible hard Brexit that risks our economy and our jobs.”
Brexit court defeat for UK government