Businessman David Abrahams has warned the government he will come out fighting if ministers start "hammering" him over the secret donations row.
Mr Abrahams says he feared accusations of a "Jewish conspiracy"
In an interview with the Jewish Chronicle, Mr Abrahams said he donated cash secretly to avoid accusations of being part of a "Jewish conspiracy".
He said his income had been "earned legitimately". No earnings came from Israel as had been alleged, he added.
Police are probing £663,975 given to Labour by Mr Abrahams through proxies.
Prime Minister Gordon Brown has said the party will pay all of the money back, and Labour officials have been holding talks with the Electoral Commission as to how this should be done.
Mr Abrahams used the newspaper interview to accuse Mr Brown's chief fundraiser, Jon Mendelsohn, of trying to "alienate" him from the Labour Friends of Israel.
Mr Abrahams said he "had helped to build up" this organisation, but Mr Mendelsohn "thought he might have opposition" when he became its chairman.
Jon Mendelsohn (right) is currently Gordon Brown's chief fundraiser (Picture: Jewish News/Jack Walker)
"He slagged me off over it and that's why I released the letter," Mr Abrahams added, referring to correspondence sent last month which allegedly contradicted Labour's claim that Mr Mendelsohn had not solicited money from Mr Abrahams.
Then he issued a warning: "If the government starts hammering me, then it might take one or two dirty turns there as well."
Mr Abrahams said some media reports about him since he was thrust into the public eye had been "character assassination, conjecture and speculation".
"The Daily Telegraph was saying that the money was not mine and that it came from Israel," he said.
"That was patently untrue. My accountant has recently done my books and it was all there.
"The money was earned legitimately through hard work and it was totally wrong to say that it came from Israel."
Mr Abrahams' donations were unlawful because people must use their own names when giving more than £5,000 to political parties.
Opposition parties have criticised Mr Brown's insistence that he knew nothing about the third-party arrangements.
Conservative leader David Cameron has said this "beggars belief", while the Liberal Democrats' acting leader, Vince Cable, called the scandal a "sleazy affair".
Asked about Mr Abrahams' latest remarks, a Labour spokesman said the party was "not commenting on matters that are part of ongoing internal and police inquiries".
The interview is published in Friday's edition of The Jewish Chronicle.