The biggest individual donor is international financier George Soros, a Hungarian-born US citizen, who has given Best for Britain £800,000 in total so far, with £400,000 of that coming since the start of the year through his pro-European Open Society Foundation.
Having made his billions in finance, including betting against sterling on Black Wednesday in 1992 - which gained him a reputation as the "man who broke the Bank of England" - Mr Soros, 87, donated more than £8bn to philanthropic causes between 1979 and 2011.
UK electoral law bans foreign donations but only in the run-up to elections or referendums.
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Best for Britain is keen to stress that it has also raised significant sums from more than 8,000 small donors.
But the involvement of Mr Soros has generated a backlash in the Brexit-backing media, with theDaily Mail telling him on its front page: "Butt out Mr Soros. You can keep your tainted money."
Mr Soros hit back at the Daily Mail's headline inan article for the Mail on Sunday, explaining why his love for Britain had led him to campaign against the "tragic mistake" of Brexit