Soros, a former refugee from communist Hungary who made $1bn (£720m) as a currency speculator betting against sterling on Black Wednesday, said leaving the EU was a tragic mistake which would weaken Britain.
The investor has come under sustained attack since details of his donations to Best for Britain were revealed last week.
Soros, who has given much of his personal fortune to his human rights organisation, the Open Society Foundations (OSF), said he had received “toxic personal criticism”. Some have said the language echoes the antisemitic attacks against him made by some governments, such as Hungary and Russia.
Writing in the Mail on Sunday, he said Best for Britain had his wholehearted support and that he had made no secret of his opposition to Brexit.
“This process will change Britain and Europe from friends into foes, at least during the transitional period,” he said. “Before the referendum Britain was doing better economically than the rest of Europe. But this has now been reversed, with continental economies powering ahead while Britain lags behind.”
Soros said he believed it was crucial to attempt to change people’s minds about leaving the EU. Referendum votes often saw one side “egged on by unscrupulous agitators” he said, adding that “people use them to express their dissatisfaction with the current state of affairs rather than contemplating the consequences”.
The 87-year-old said he was also concerned about the divisions between rich and poor over Brexit. “Old voters have overruled young voters who will have to live with the consequences of Brexit for decades ahead. This is aggravating the disillusionment with democracy among young people,” he said.
“But a mere reversal of the 52:48 majority for Brexit is not enough. The majority for staying would have to be significantly larger to convince Europe that Britain’s attitude towards Europe has fundamentally changed and its decision deserves to be taken seriously.”
Soros said he believed public opinion was slowly shifting in Britain, but that time was running out and a tipping point needed to be reached in the next six to nine months.
“Best for Britain seeks to break the logjam by bringing together all the various forces that are united by their aspiration for Britain to remain part of Europe. It has my wholehearted support,” he said.
Best for Britain, which advocates remaining in the EU rather than fighting for a soft Brexit, received the six-figure sum from OSF since the June 2017 election. The campaign is chaired by Mark Malloch-Brown, a former UK government minister and deputy UN secretary general.
It was co-founded by Gina Miller, who took the UK government to court over the triggering of the article 50 process to leave the EU. Miller has since said she is no longer part of the organisation, which she has accused of being undemocratic.
Lord Malloch-Brown said the campaign group had followed all rules and regulations governing financial contribution. “We, like millions of people, believe that Britain should lead, not leave, Europe,” he said.
“We work with campaigners, businesses, unions, politicians and community groups to make sure everyone has a strong campaigning voice,” he said.