The creator of the Article 50 process by which the UK leaves the European Union has told BBC Wales the UK can - and should - reverse the decision.
Lord Kerr of Kinlochard - a former UK ambassador to the EU - drafted the text in the early 2000s.
The UK government has insisted that Article 50 cannot be reversed and it had no wish to do so.
Lord Kerr told Sunday Politics Wales an election or another referendum was needed before the Brexit process ended.
"We are members of the European Union until the moment we leave," Lord Kerr said.
"During the two years [of the Brexit process] - or its extension - it is open to you to say, 'actually we've now changed our mind, do you mind if we stay?'
"Whether they agreed would be a political question of course... they might try to extract some sort of price from us if we'd wasted a lot of their time."
Lord Kerr confirmed his own view that the UK should remain in the EU.
"I would like to believe that it is still possible for the country to change its mind when it sees that it was, in the referendum campaign, told a lot of things that weren't actually true by those campaigning to leave," he said.
"When it sees the economy is slowing down as a result of the referendum decision, I would hope that the country might feel that this question needs to be looked at again."
Asked how he thought the reversal of Article 50 should happen, Lord Kerr said: "Your guess is as good as mine.
"I'm fairly expert at the legal position in Brussels and what would happen if we did change our minds.
"The process, in this country, of getting a change of mind I find much harder to understand.
"I think it probably is the case that it would need a general election, or another referendum, or both, in order to be clear the country had changed its mind."
Lord Kerr also raised concerns that the UK government had not done enough to gain support from the devolved administrations, including the Welsh Government.
The former diplomat was speaking ahead of a conference in Hay-on-Wye, Powys, marking a year since the UK voted to leave the European Union.
The UK government's Department for Exiting the EU has been asked to respond to Lord Kerr's comments.