Parliament must vote on whether the UK can start the process of leaving the European Union, the High Court has ruled.
This means the government cannot trigger Article 50 of the Lisbon Treaty - beginning formal discussions with the EU - on its own.
Theresa May says the Brexit referendum and ministerial powers mean MPs do not need to vote, but campaigners argue this is unconstitutional.
The government is expected to appeal.
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. This follows the UK's decision to back Brexit in June's referendum by a margin of 51.9% to 48.1%.
The EU's other 27 members have said negotiations about the terms of the UK's exit - due to last two years - cannot begin until Article 50 has been invoked.
BBC assistant political editor Norman Smith said, if the court's decision is not overturned, there could be "months and months" of parliamentary hurdles ahead.
The government is expected to appeal against the decision, with a further hearing to be held in the Supreme Court.