The man who wrote Article 50 says Brexit could be reversed
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Once Britain has triggered Article 50 of the EU's 2007 Lisbon Treaty, it has two years to complete negotiations and formally leave the union.
Many lawyers have argued that the decision to leave is irreversible once that process is underway, but Lord Kerr of Kinlochard told BBC News that "you can change your mind while the process is going on."
He said: "During that [two year] 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.
Lord Kerr's comments came on Wednesday, a day before the British government lost a High Court case which determined that Prime Minister Theresa May will now be forced to seek the approval of parliament before she triggers Article 50.
The case, which was led by investment banker Gina Miller, rejected the government's argument that it could use the legal mechanism of royal prerogative to allow May to trigger Article 50 without receiving the consent of parliament first.
But confusion still remains over the finer legal details of Article 50 once it has actually been triggered, which Theresa May says will happen by March 2017.
The question of whether Article 50 is reversible or not is crucial. If Article 50 can be reversed, then Britain would have significantly more bargaining power with Brussels, as it would allow the country to walk away from negotiations if they were unfavourable, and start again at a time of their choosing.
On the other hand, if Article 50 cannot be reversed, then Brussels would have far more bargaining power in negotiations, as Britain would face accepting whatever terms it was offered or leaving the EU without a deal, knowing that it only had two years to complete the exit process.
Lord Kerr's words echo legal advice given to the House of Lords earlier in October, as well as comments from former director-general of the Council of the European Union's Legal Service Jean-Claude Piris, who said in September that "even after triggering Article 50 ... there is no legal obstacle to the UK changing its mind."
He admitted that he thought the 262-word clause, written when he was general secretary of the European Convention in 2002 and 2003, would only ever be used in the event of a "coup."
He said: "I thought the circumstances in which it would be used, if 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 added.
A spokesperson for the government's Brexit department, the Department for Exiting the European Union, told the BBC: "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."
The government has now announced that it will challenge the High Court's decision to make the government consult parliament before triggering Article 50, meaning that an appeal will be heard in the UK Supreme Court.