A campaign to secure a second Brexit referendum is to be launched in days, the billionaire George Soros has announced.
The financier, in a keynote speech, has said the prospect of the UK's prolonged divorce from Brussels could help persuade the British public by a "convincing margin" that EU membership was in their interests.
Mr Soros is reported to have given about £500,000 to Best for Britain, which was set up last year by anti-Brexit campaigner Gina Miller.
The group is expected to publish its campaign manifesto on June 8, calling for a second referendum.
Speaking in Paris, the 87-year-old said: "Brexit is an immensely damaging process, harmful to both sides.
"Divorce will be a long process, probably taking more than five years. Five years is an eternity in politics, especially in revolutionary times like the present.
"Ultimately, it's up to the British people to decide what they want to do. It would be better however if they came to a decision sooner rather than later. That's the goal of an initiative called the Best for Britain, which I support.
"Best for Britain fought for, and helped to win, a meaningful parliamentary vote which includes the option of not leaving at all. This would be good for Britain but would also render Europe a great service by rescinding Brexit and not creating a hard-to-fill hole in the European budget.
"But the British public must express its support by a convincing margin in order to be taken seriously by Europe. That's what Best for Britain is aiming for by engaging the electorate. It will publish its manifesto in the next few days."
Mr Soros said he feared the EU could be heading towards another major financial crisis triggered by austerity and populist political parties intent on blowing the bloc apart.
"The EU is in an existential crisis. Everything that could go wrong has gone wrong," he said.
However, Mr Soros said he was convinced it was the ideal time for the EU to reform itself and prepare the ground for the UK staying inside the bloc.
"The economic case for remaining a member of the EU is strong, but it will take time for it to sink in", he added.
"During that time the EU needs to transform itself into an association that countries like Britain would want to join, in order to strengthen the political case.
"Such a Europe would differ from the current arrangements in two key respects. First, it would clearly distinguish between the European Union and the eurozone.
"Second, it would recognise that the euro has many unresolved problems and they must not be allowed to destroy the European Union."
Prime Minister Theresa May is committed to leaving the EU's single market and customs union after Brexit, which officially takes place on March 29 next year, although a transition period is currently set to last until December 31, 2020.