European Council President Donald Tusk said he sees the chances of Britain staying in the European Union after all at 20-30%, as Britons had begun a genuine debate whether to leave or to remain only after the 2016 Brexit referendum.
In a Polish newspaper interview, Mr Tusk suggested holding another Brexit referendum given the precedent that Britain had already staged two plebiscites on Europe, first in 1975 on whether to stay in the then-European Economic Community that it had joined two years before.
If in 2016 it was possible to change the decision made in 1975, it should be possible to hold a second Brexit referendum, said Mr Tusk, who has often voiced sadness at the outcome of the vote in which 52% voted to leave against 48% to remain.
"The real debate on the consequences of Brexit started not before or during the referendum campaign, but after the vote. Today the results would probably look different," he said in remarks published on the Gazeta Wyborcza daily website.
"Paradoxically it is Brexit that triggered a pro-European movement in the UK," Mr Tusk said. "Today, chances that there will be no Brexit are at 20-30%. That's a lot."
He was touching on growing uncertainty over how, when or indeed even if Britain quits the EU.
Prime Minister Theresa May has thrice failed to get her EU withdrawal agreement ratified by the British parliament so the EU allowed a Brexit delay until the end of October, giving her more time to try to win over MPs.
But prospects for overcoming the impasse soon appear dim, with both her Conservative and main opposition Labour parties split into multiple shades of pro-Leave and pro-Remain factions, and Mrs May facing possible leadership challenges.
Opinion polls over the past six months have hinted at a shift towards a narrow pro-Remain majority, as public debate has aired the economic and political complications of leaving the EU, though analysts say another vote could go either way.
Britain will take part in European Parliament elections later this month as a result of having Brexit delayed until 31 October. Some see the vote as a new referendum on Brexit, one whose outcome could help block withdrawal - or accelerate it - as the debate on the pros and cons of the move churns on.