The funding for the Best for Britain campaign came through one of Mr Soros’s foundations, but anti-Brexit group chairman, Lord Malloch-Brown, a former minister, and diplomat, said the group had followed rules governing financial contributions “to the letter”.
He said: “We have never hidden our agenda; we have been campaigning hard to win a meaningful vote on Brexit, which we did, and to keep all options on the table, including staying in the European Union.
“We, like millions of people, believe that Britain should lead, not leave Europe.
“We work with campaigners, businesses, unions, politicians and community groups to make sure everyone has a strong campaigning voice.
“George Soros’s foundations have, along with a number of other major donors, also made significant contributions to our work.
“Indeed through his foundations he has contributed £400,000.
“But our small donors have collectively contributed more and the commitments of the other major donors also exceed this amount.
“So he is an important and valued donor but his funding is one amongst many sources.”
Best for Britain was founded by Gina Miller to fund election campaigns for anti-Brexit MPs, but she stepped down as director of the group after the 2017 general election.
Mr Soros is one of the world’s richest men who is known as the man who “broke the Bank of England”.