The court however ruled the government need not consult the assemblies of Scotland, Northern Ireland and Wales before beginning Brexit negotiations.
The UK Supreme Court has ruled that Prime Minister Theresa May cannot use executive powers to start the process of Britain's exit from the European Union without seeking prior approval from parliament.
The court voted 8 to 3 on Tuesday that the government cannot trigger Article 50 of the EU's 2009 Lisbon Treaty – by which Britain notifies the EU of its intention to leave the bloc – without parliamentary approval. The Supreme Court also decided the agreement of the devolved legislatures of Scotland, Wales and Northern Ireland was not required before the government begins Brexit negotiations.
Lord David Neuberger, president of the Supreme Court, presented the summary of the full court's decision.
The British government responded later in the day, saying it will introduce "straightforward" legislation within days seeking parliament's approval to trigger Britain's divorce with the EU.
"This will be the most straightforward bill possible to give effect to the decision of the people and respect the Supreme Court's judgment," Brexit minister David Davis said, addressing parliament.
Simon McGregor-Wood reports.
Why the Supreme Court was involved
Britons voted on June 23, 2016 to leave the European Union. Prime Minister May intended to invoke Article 50 by the end of March, beginning a two-year divorce process.
The government had argued it could take the action without lawmakers' approval, using a historical power known as "royal prerogative."
Those challenging May, led by investment fund manager Gina Miller and backed by the Scottish government, said under Britain's unwritten constitution May must first get lawmakers' approval as leaving the EU will strip Britons of rights they were granted by parliament. That view was backed by London's High Court in November, prompting the government to appeal to the Supreme Court, the highest judicial body in the land.
With this loss, May will probably need to bring in a parliamentary bill that will open up the Brexit process to scrutiny from lawmakers. A clear majority of lawmakers backed staying in the EU ahead of June's referendum in which Britons voted by 52-48 percent to leave.