Theresa May intends to notify the EU of Britain’s intention to withdraw on March 29, triggering two years of negotiations that would end with Brexit in 2019.
She announced at the Conservative party conference in October that Article 50 of the Lisbon treaty will be triggered “no later that the end of March."
Is Article 50 irrevocable?
The Government argues that the decision to trigger Article 50 is irreversible because David Cameron promised to act on will of voters and respect the outcome of June's referendum.
The question of irrevocability was raised during a Supreme Court hearing on the right of Parliament to give its approval to trigger the process of leaving the EU.
Jeremy Wright QC, who represented Theresa May, argued:
We do not argue that an Article 50 notice can be revoked and we would like the court to proceed on the basis a notification is irrevocable.
He added that "parliament's role in the process" of leaving the EU does not just stop after Article 50 is triggered.
At the time a Department for Exiting the European Union spokesman said: "The UK voted to leave the European Union. The people have spoken and it is now the duty of the government to make sure that happens.
"Government lawyers also made clear in legal proceedings before the High Court that, as a matter of firm policy, notification of withdrawal will not be withdrawn."
Who argues that it is?
Lord Kerr, who devised the clause in the Lisbon Treaty, has argued the country “might want to think again” when the details of the Prime Minister's deal with the EU emerge.
Speaking to the BBC, he said:
It is not irrevocable. You can change your mind while the process is going on.
During that period, if a country were to decide actually we don't want to leave after all, everybody would be very cross about it being a waste of time.
They might try to extract a political price but legally they couldn't insist that you leave.
He added that he believed the circumstances in which it would be used, is ever, would be when there was a coup in a member state and the EU suspended that country's membership.
"I thought that at that point the dictator in question might be so cross that he'd say 'right, I'm off' and it would be good to have a procedure under which he could leave", he said.