Leave.EU chairman Arron Banks has unleashed a broadside against the Electoral Commission, accusing the democracy watchdog of being a "swamp creature" packed with "fully signed-up Remainers".
The Brexit-backing businessman is facing a commission investigation into whether he breached campaign finance rules in last year's EU referendum.
The commission has said it is looking into whether Mr Banks was the "true source" of three loans worth £6 million on non-commercial terms to Leave.EU, and whether Better for the Country Limited (BFTCL) - a company that lists him as a director - was acting as an "agent" when it donated £2.3 million to five registered campaigners.
In a letter to commission chief executive Claire Bassett, Mr Banks repeated his assurance that Leave.EU was funded by himself, financier Peter Hargreaves and public donations.
He insisted that "not one shred of evidence" had been produced to back allegations of "dark Russian money" coming into the campaign.
He restated his call for a judge-led inquiry to oversee the funding of all the referendum campaigns, and questioned why the commission - which has a separate investigation under way into the alleged undeclared provision of services to Leave.EU by data firm Cambridge Analytica - was only "targeting" Leave.EU.
Mr Banks wrote: "There is a concerted political effort by Remain MPs to claim people were misled by 'numbers on the side of a bus' or that the campaign was funded by cash from Russia or somewhere else overseas. I am proud of the fact that the Leave.EU campaign was funded by myself, Peter Hargreaves and donations from the British public in huge numbers."
Mr Banks accused the commission of "collaborating" with Remain-backing Labour MP Ben Bradshaw, who raised questions in the House of Commons about alleged Russian interference in the democratic processes of Western countries.
He asked whether the watchdog regarded contributions to the Remain campaign by large banks and corporations as "worthy of investigation" and suggested that bodies including the Bank of England and CBI had effectively contributed to the In camp through warnings they produced about the possible economic damage of Brexit.
Describing the commission as "a swamp creature created by New Labour and packed full of the flotsam of British politics", Mr Banks said: "Ten out of ten of the Commissioners are fully signed-up Remainers with links to EU thinktanks. One sits on the same committee as (former Swedish prime minister) Carl Bildt and two ex-European Commissioners.
"It really is quite incredible that an organisation that exists to ensure fair play in elections should be packed full of ex-MPs and ex-council leaders - jobs for the political boys."
An Electoral Commission spokesman said: "We have received Mr Banks' letter and will respond directly to him in due course."