England's High Court of Justice ruled on Thursday that the UK Parliament must vote in order to trigger the process taking the country out of the European Union, causing a major upset to Prime Minister Theresa May's plans for Brexit.
May intends to trigger Article 50, a provision in the EU's Lisbon Treaty that deals with the mechanism for leaving the bloc, between the start of the next year and the end of March to allow the government time to prepare its negotiating position.
The court granted the government permission to appeal against the ruling and a government lawyer said the Supreme Court had set aside the dates of December 5 to 8 to hear the matter.
Three senior judges said May's government does not have the power by itself to trigger Article 50 of the EU's Lisbon Treaty, the formal notification of Britain's intention to leave the bloc.
"We hold that the secretary of state does not have power under the crown's prerogative to give notice pursuant to Article 50... for the United Kingdom to withdraw from the European Union," the judgment said.
"The court does not accept the argument put forward by the government," said Lord Chief Justice John Thomas, reading out the three judges' ruling.
A government spokesperson said in a statement, "The country voted to leave the European Union in a referendum approved by Act of Parliament
"The government is determined to respect the result of the referendum. We will appeal this judgement."
International Trade Secretary Liam Fox said the government was disappointed with the court ruling and would consider it carefully before deciding how to proceed.
The prime minister's spokesperson said that the government would continue working towards the UK leaving the EU despite the court ruling.
"Our plan remains to invoke Article 50 by the end of March, we believe the legal timetable should allow for that," she told reporters.
"We have no intention of letting this derail our timetable."