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.
The funding for the Best for Britain campaign came through one of Mr Soros’s foundations
“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”.
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He made a billion dollars betting against sterling on Black Wednesday in 1992.
Black Wednesday was a result of the UK joining the European Exchange Rate mechanism – a precursor to the Euro.
Mr Soros saw the unfavourable position of the UK while joining and made billions after the UK lost £3.4billion in a single day.
The billionaire previously said it was possible the UK would apply to rejoin the European Union soon after Brexit.
Theresa May’s former chief of staff Nick Timothy, who backed the Leave cause, used his newspaper column to claim that Best for Britain wanted to bring down Mrs May’s Government.
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In the Telegraph he wrote: “The objective is to convince MPs to vote against the deal Theresa May negotiates with Brussels, regardless of its content and despite the risk that doing so could mean Britain leaves the EU with no alternative agreement in place.
“Malloch-Brown and his backers believe that, if Parliament rejects the Brexit deal, the Government will fall, and Brexit can then be stopped.”
He said it should be a “wake-up call” to Conservatives in Parliament, adding: “However Tory MPs feel about Brexit, voting against the Government when the deal is done risks something much, much worse.”
The article has sparked outrage on social media, with Guardian columnist Owen Jones accusing the piece of being “anti-semitic”.
Nick Timothy claimed Best for Britain wanted to bring down the Government
He said: “The lead Telegraph story, jointly penned by Theresa May’s ex-chief of staff, alleges a conspiracy led by George Soros against Brexit. It uses a smear campaign by Hungary’s dictatorship as evidence.
“Call it out for what it is: dog-whistle anti-Semitism.”
Mr Soros was accused by the Hungarian Government of being behind a plan to flood the EU with cheap migrant labour.
The article says Mr Soros was accused of toppling Governments, but he it does not mention that it was by Russia.
Lord Malloch-Brown said the British people deserve a ‘final say’
Lecturer for German and European Studies at King’s College London Alexander Clarkson said: “Soros has been used as a scapegoat by Orban’s Fidesz, Erdogan’s AKP, the Kremlin, Milosevic and the GOP. Does the British Conservative Party really want to join that list?”
In response to the accusations, Nick Timothy said on Twitter: “I haven’t written about Hungary… If you read my column you will see I have written about the objectives of the anti-Brexit campaign and nothing else.”
In response to the article, Lord Malloch-Brown said: “We think the British people deserve a final say on the Brexit deal and believe the country has been led down a dangerous false turn.
“This is a democratic and patriotic effort to recover our future and we welcome support for our efforts from many quarters.”
A Best of Britain source said the story was crazy but delighted with the profile it gave them.
The source added: “I’m well happy.”
Express.co.uk has contacted Mr Soros, the Open Society Foundations and Best for Britain for comment.