Billionaire investor George Soros has handed £400,000 to a campaign arguing against Brexit.
The funding for Best for Britain came through one of Mr Soros's foundations.
Best for Britain 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."
Mr Soros, one of the world's richest men who made a billion dollars betting against sterling on Black Wednesday in 1992, previously said it was possible that the UK would apply to rejoin the European Union soon after Brexit.
The involvement of Mr Soros was reported in the Daily Telegraph in a story written by a team including Theresa May's former chief of staff Nick Timothy.
Mr Timothy, who backed the Leave cause, used his column in the newspaper to claim that Best for Britain wanted to bring down Mrs May's government.
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."
But 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."