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 exit negotiations with the EU – on its 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.
A statement is to be made to MPs on Monday but the prime minister’s official spokesman said the government had “no intention of letting” the judgement “derail Article 50 or the timetable we have set out. We are determined to continue with our plan”.
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”.
And UKIP leader Nigel Farage said he feared a “betrayal” of the 51.9% of voters who backed leaving the EU in June’s referendum and voiced concern at the prospect of a “half Brexit”.
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, although a majority of MPs had backed the Remain campaign, most would ultimately be likely to vote for Article 50, as Brexit had been supported 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.
Investment manager 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 in 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.”
Government lawyers had argued that prerogative powers were a legitimate way to give effect “to the will of the people”.