Brexit may not be inevitable, according to the crossbench peer who wrote Article 50 of the Lisbon Treaty, the clause by which Britain will leave the European Union.
Lord Kerr of Kinlochard said he believed Britain could still stay in the EU after exit negotiations start.
"It is not irrevocable,” he told the BBC. "You can change your mind while the process is going on."
His interview with the BBC comes ahead of a high court ruling today to determine if the government can begin the procedure to leave the EU without consulting parliament.
Prime minister Theresa May has said she wants to trigger the UK’s exit by March 2017.
But last month judges heard a challenge from campaigners who claimed May did not have the power to invoke Article 50 without MPs' approval.
Kerr, who is advising Scotland's first minister Nicola Sturgeon on Brexit, continued: "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."
Kerr also renewed calls for either parliament or the British public to be given a second chance to stop Brexit, saying the country “might want to think again” once the terms of the UK’s exit become clear.
The crossbench peer went on to say he had only devised Article 50 in case of a coup in a member state: "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."
Following his remarks, a spokesperson from the the department for exiting the European Union told the BBC the “people had spoken” regarding the UK’s exit from the EU.
"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in legal proceedings before the high court that, as a matter of firm policy, notification of withdrawal will not be withdrawn," they said.