The Scottish cross-bench peer who wrote Article 50 – the procedure by which the UK would leave the EU – believed it was “not irrevocable”.
In a BBC interview, Lord Kerr of Kinlochard said the UK could choose to stay in the EU even after exit negotiations had begun.
He has also renewed calls for either parliament or the public to be given a chance to stop Brexit.
The UK government insisted the leave vote must be respected.
His comments come as the High Court is to rule on whether the government can begin the formal process of leaving the European Union without consulting Parliament.
Prime Minister Theresa May said she would trigger the two year exit negotiation process by the end of March 2017.
This procedure, which is described in Article 50 of the Lisbon treaty, should ensure the UK leaves the EU in 2019.
But Lord Kerr, who devised the clause, said the country “might want to think again” when Brexit terms become clearer.
He explained: “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.”
Different arrangements possible
Lord Kerr wants either parliament or the public – through an election or a second referendum – to revisit the decision to leave the EU in a year to 18 months time.
Mrs May has repeatedly ruled out reversing the result of the EU referendum vote which took place on 23 June and resulted in 52% of UK voters backing the campaign to leave.
Lord Kerr is advising First Minister of Scotland Nicola Sturgeon on Brexit as she seeks to maintain Scotland’s links with the EU.
He does not think it would be possible for Scotland to remain in the EU single market if the UK as a whole is leaving.
However, he said different arrangements for access to the single market for different parts of the UK could be possible.
Lord Kerr said: “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.”
The former diplomat also said that he does not believe a vote for independence would secure Scotland unbroken 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,” he said.
When Lord Kerr argued a similar case during the 2014 Scottish independence referendum campaign, the Scottish government insisted that the EU would find a way of accommodating Scotland.
Although the UK as a whole chose to leave the EU in the referendum ballot, voters in Scotland decided by 62% to 38% to stay.
Lord Kerr is a former head of the British diplomatic service and served as both UK ambassador to the EU and the United States.
‘I thought only a coup would trigger it’
After leaving the foreign office, he 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.
Lord Kerr said the process “wasn’t really necessary” because it has always been possible for a country to quit.
He added: “If you stopped paying the bills and you stopped turning up at the meetings, in due course your friends would notice that you seemed to have left.”
Lord Kerr said he had never imagined the UK would make use of Article 50.
He explained: “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.”
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.”
Article 50 author Lord Kerr says Brexit not inevitable}