The High Court has ruled that ministers cannot begin the process of leaving the European Union without a vote in Parliament.
Despite the referendum result in June, the court ruled that the Government does not have the power to trigger Article 50 of the Lisbon Treaty by itself.
Theresa May became prime minister after the EU referendum
The Government has been given permission to appeal to the Supreme Court and a hearing is expected to take place in early December.
Prime minister Theresa May had previously said she would trigger Article 50 by the end of March next year but the ruling, if it is upheld, could delay that.
The lord chief justice said: ‘the most fundamental rule of the UK constitution is that parliament is sovereign.’
A Government spokesman said: ‘The country voted to leave the European Union in a referendum approved by Act of Parliament. And the Government is determined to respect the result of the referendum. We will appeal this judgment.’