Businessman Arron Banks has been challenged to explain where the money for his Brexit campaign came from.
Mr Banks is the subject of a police probe into claims he was not the "true source" of £8m loaned to his Leave.EU campaign at the 2016 EU referendum.
The Electoral Commission referred the case to the National Crime Agency, which said potential criminal offences may have been committed.
Mr Banks told the BBC's Andrew Marr the money came from his UK businesses.
Foreign donations to political campaigns are banned under UK law.
Mr Banks has faced claims that money to fund Leave.EU came from Russian sources - something he has repeatedly denied.
"There was no Russian money and no interference of any type. I want to be absolutely clear about that," he told the BBC's Andrew Marr show.
"The money came from Rock Services which was a UK limited company."
Who is Arron Banks?
- The Bristol-based insurance tycoon, who was born in Cheshire, was a Lloyd's underwriter before starting his own firms
- The twice married father-of-five has been a close friend and supporter of Nigel Farage
- Mr Banks was a Conservative supporter until 2014 when he defected to UKIP, giving them £1m
- He is said to be one of the biggest political donors in UK history, thought to have given up to £9.6m to Leave.EU - which he co-founded - and UKIP
- Leave.EU, which was backed by then UKIP leader Mr Farage, lost out to Vote Leave in the battle to become the official Leave campaign in the 2016 EU referendum
- His Leave.EU campaign targeted its message at voters angry about EU immigration
The Electoral Commission says it suspects the money came from Rock Holdings, the parent company of Rock Services, which is registered in the Isle of Man and so "could not lawfully make any donation or be a party to any loan to Leave.EU".
The Commission referred the case to the National Crime Agency because it has the ability to investigate financial transactions in the Isle of Man, something that is beyond the scope of the electoral watchdog.
Mr Banks denied Rock Services was a "shell company" that did not have sufficient funds to be the source of the money for Better for the Country (BFTC), which ran Leave.EU.
In its report on the case, the Electoral Commission said: "We have reasonable grounds to suspect that Rock Services did not fund the payments of £8m it is said it made to BFTC from its UK-generated income".
Mr Banks said Rock Services has "all sorts of revenues" but did not detail them.
He said: "We insure half a million people.
"I know it's complex for journalists to understand but we know what this is about - it's about undermining Article 50 and the Brexit result.
"It's a group of vicious MPs who have grouped together with the Guardian and the FT."
Mr Banks was also asked about a report in the Sunday Times that he would back Remain if the 2016 referendum were re-run.
He replied: "What I said was that the corruption I have seen in British politics, the sewer that exists and the disgraceful behaviour of the government over what they are doing with Brexit and how they are selling out, means that if I had my time again I think we would have been better to probably remain and not unleash these demons."
Mr Banks has been accused of misleading Parliament over links between Leave.EU and his insurance businesses.
The Observer says it has seen internal emails leaked by former employees of Eldon Insurance, another of Mr Banks's companies, and Rock Services that show they worked on the Leave.EU campaign from their company offices.
Conservative MP and chairman of the culture media and sport committee, Damian Collins, told The Observer the leaked e-mails appeared to "flatly contradict" what Mr Banks had told his committee in June.
Mr Banks told Andrew Marr: "I can say that was reported to the Electoral Commission, and people who did work for Eldon were transferred over on short-term contracts legally, and it was reported through the Electoral Commission in the right way.
"You are talking about emails that were stolen from us."
The Electoral Commission said Leave.EU "did not report spending on or donations from Eldon".
Mr Banks was asked by Andrew Marr why he wrote to Mr Collins' constituents calling him a "snake in the grass".
Mr Banks replied: "Well he is."
Mr Collins responded to the letter last week by saying he was "not going to be intimidated out of doing my job" by Mr Banks.
On Tuesday, the Information Commissioner, Elizabeth Denham, will announce the outcome of an inquiry into claims Eldon shared customer data obtained for insurance purposes with Leave.EU, potentially breaking data protection laws.
"We are also investigating whether Eldon Insurance Limited's call centre staff used customer databases to make calls on behalf of Leave.EU in contravention of the Privacy and Electronic Communication Regulations 2003," the ICO said in an interim report in July.