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".
Brexit Secretary David Davis said he presumed the court ruling meant an act of Parliament would be required to trigger Article 50 - so would be subject to approval by both MPs and peers.
But the government was going to contest that view in an appeal, saying that the referendum was held only following "a six-to-one vote in the Commons to give the decision to the British people".
"The people are the ones Parliament represents - 17.4m of them, the biggest mandate in history, voted for us to leave the European Union. We are going to deliver on that mandate in the best way possible for the British national interest," he told the BBC.
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".
But 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