The Scottish peer who wrote Article 50 — the procedure by which the UK would leave the EU — told the BBC in an interview he believed the process was “not irrevocable.”
Lord Kerr of Kinlochard said the UK could choose to stay in the EU even after exit negotiations had begun.
He repeated his calls for either parliament or the public to be given a chance to stop Brexit.
Kerr is a former head of the British diplomatic service and served as both UK ambassador to the EU and the United States.
Kerr is advising Scotland’s First Minister Nicola Sturgeon on Brexit as she seeks to maintain Scotland’s links with the EU.
UK Prime Minister Theresa May has said she would trigger the two year exit negotiation process by the end of March 2017.
Kerr, who devised Article 50, said the UK “might want to think again” when Brexit terms become clearer.
“It is not irrevocable,” said Kerr.
“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.”
The BBC reported that Kerr wants either the UK parliament or the UK public — through an election or a second referendum — to revisit the decision to leave the EU in the next 12 to 18 months.
Kerr does not think it would be possible for Scotland to remain in the EU single market if the UK ends up leaving.
He said however that different arrangements for access to the single market for different parts of the UK could be possible.
“It is possible to envisage the Scots being in an ante room to the council (of EU ministers) in Brussels, rather closer to the action than the English might be on particular subjects,” said Kerr.
Kerr said he does not believe a vote for independence would secure Scotland continued membership of the EU.
“I think that when independent the Scots could apply and probably get in pretty quickly through the door marked accession. But they can’t stay inside,” said Kerr.
After leaving the UK foreign office, Kerr was secretary-general of the European convention, which drafted what became the Lisbon treaty.
It included Article 50 which sets out the process by which any member state can leave the EU.