The Supreme Court ruled by a majority of eight to three that Parliament must decide whether the government can start the Brexit process.
Premier Theresa May cannot start talks about Britain leaving the European Union without the approval of members of both houses of parliament.
The Supreme Court unanimously ruled that the Scottish Parliament and Welsh and Northern Ireland assemblies are not required to have a similar vote.
The ruling may not slow down the UK’s move to leave the European Union.
After last June’s 52-48 result in the EU referendum, most MPs are thought unlikely to vote to block a Brexit bill and stop Theresa May from triggering her Article 50 Brexit gun before her self-imposed deadline of March 31.
Labour leader Jeremy Corbyn said his party would not “frustrate the process for invoking Article 50” but will seek to amend the Government bill.
Labour’s Brexit spokesman, Sir Kier Starmer, said his party will try to secure a guarantee from Mrs May that the vote she has promised for 2019 on the shape of the final terms of exit from the EU, and the future relationship, will be “meaningful”.
Court president Lord Neuberger said:
By a majority of eight to three, the Supreme Court today rules that the Government cannot trigger Article 50 without an Act of Parliament authorising it to do so.
The Government failed in its appeal against a November High Court ruling that the prime minister cannot trigger Brexit by using the ancient royal prerogative power to make and break foreign treaties.
Mrs May has already outlined a vision for a “hard” Brexit, which includes pulling out of the single market.
Attorney General Jeremy Wright said the government was “disappointed” but would “comply” and do “all that is necessary” to implement the court’s judgement.
Gina Miller, one of the campaigners who brought the case against the government, said Brexit was “the most divisive issue of a generation”, and said her victory was “not about politics, but process”.