Soros, 87, founder of the Open Society Foundation, will work with the pro-EU Best for Britain to change the minds of the British public and convince them that EU membership is in their best interests, he said in a speech.
The campaign, to be launched in the next few days, will aim to secure a second Brexit referendum within a year.
"Brexit is an immensely damaging process, harmful to both sides ... Divorce will be a long process, probably taking more than five years," said Soros, calling this period "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," he added.
Best for Britain, set up shortly after the 2016 EU referendum, has successfully managed to lobby MPs in parliament to vote against the government's proposed EU Withdrawal Bill and introduce amendments, such as staying in the single market and Customs Union and ensuring that March 2019 is not the official exit date.
The government was defeated 17 times in the House of Lords, parliament's upper chamber, when it brought the EU Withdrawal Bill to vote and is expected to face the same fate when the bill reaches the House of Commons.
"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," the billionaire said.
The U.K. is set to leave the EU in March 2019. The transition period, however, in which the U.K. will continue to be a part of the single market and the Customs Union, is expected to last through December 2020.
