The Hungarian financier and philanthropist who is believed to have donated £500,000 to support efforts to secure a referendum on the UK’s Brexit deal said leaving the EU would be an “immensely damaging process”.
Mr Soros is supporting the Best for Britain campaign, which will launch a manifesto for a new referendum on the EU on 8 June.
In a speech in Paris on Tuesday night, he said the prospect of a prolonged divorce from Brussels would persuade the British public by a "convincing margin" to stay in the EU
"Brexit is an harmful to both sides,” the 87 year-old said. "Divorce will be a long process, probably taking more than five years.
"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.”
Mr Soros added: "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, who has been the target of anti-Semitic attacks for his active campaigning on political issues across the world, admitted that the EU was “in an existential crisis” and said “everything that could go wrong has gone wrong" in the bloc.
However, he said the EU could still reform itself, with the UK as a member. “The EU needs to transform itself into an association that countries like Britain would want to join, in order to strengthen the political case,” he said.
"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."
Speaking on the BBC’s Today programme on Wednesday, Former foreign office minister Lord Malloch-Brown, the head of Best for Britain, said the UK needs to stay close to the EU because a policy of appeasement in the 1930s showed the consequences of being cut off from the continent.
"Britain's history as an island nation adjacent to mainland Europe is when we try to, sort of, pull away from Europe's problems and close ourselves off to them they have a horrible habit of infecting us anyway,” he said.
"Appeasement in the 1930s, you name it. For centuries Britain has ignored events on continental Europe at its peril."