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.
UKIP leader Nigel Farage said he feared a “betrayal” of the 51.9% of voters who backed Brexit 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.
But he said 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.”
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 verdict, 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.”