Mark Malloch-Brown, chairman of the pro-EU Best for Britain campaign announced the investment by Soros on Thursday, amid accusations that his group had joined with the Jewish investor to "plot to thwart Brexit", according to presstv.ir.
"Through his foundations he has contributed £400,000 (453,000 euros, $555,000)," said Malloch-Brown, who served as a former minister between 2007 and 2009.
Pro-Brexit newspaper, The Daily Telegraph, said in a report that the business magnate was also planning UK-wide advertisements this month "which they hope will lead to a second referendum to keep Britain in the EU."
Defending his campaign, Malloch-Brown said it was "perfectly reasonable" for the Members of Parliament to be able to rethink Brexit.
"There are a lot of people out there who are frustrated," he told BBC. "A majority of MPs are still personally Remainers and yet are about to vote for anything between a hard and a soft Brexit."
Soros, 87, has been known for providing financial support to opposition movement around the globe from the Russian capital Moscow to his own country of origin Hungary.
He became a hate figure in Britain after “breaking the Bank of England” in 1992, where he made billion dollar battings against sterling on Black Wednesday and forced Britain to withdraw the pound from the European Exchange Rate Mechanism (ERM).
Soros has made no effort to hide his disregard for Brexit, a decision that 52 percent of Britons made during a referendum in 2016 in a bid to create more jobs by breaking free from the bloc’s regulations and stopping immigration into the UK.
In a speech at the World Economic Forum in Davos last month, Soros predicted that British Prime Minister Theresa May, who has so far failed to put forth a meaningful Brexit roadmap, would not last long in power.
He also said British people were "in denial" about the financial implications of leaving the EU.
"The current economic situation is not as bad as it was predicted, they live in hope, but as the currency depreciates, and inflation will be the driving force, that will lead to declining living standards,” he said.
"It's going to take some time but when it does happen, they will realize that they are earning less than before, because wages won't rise as fast as the cost of living," Soros added.
Experts have traced Soros’ influence over political into the US, where he is accused of funding activist groups to stage widespread protests against President Donald Trump.