Gordon Brown’s chief fundraiser must have been a major donor was dodging electoral rules to keep his identity secret, opposition lawmakers say, calling for his resignation.
Jon Mendelsohn, drafted by Brown to fund a possible autumn general election campaign, sent a letter to the backer, seemingly aware of large donations to Brown's Labour party.
Businessman David Abrahams gave Labour around 650,000 pounds (US$1.3million; EUR905,000) in the names of other people to avoid laws that require major donors to be named.
Some of those middlemen have since claimed they were unaware Abrahams was using their details of channel the funds.
Abrahams said late Tuesday he had received a handwritten note from Mendelsohn on Monday, praising him as one of the party's strongest supporters.
"If Jon Mendelsohn - this is Gordon Brown's personal fundraiser, the man he brought in to fund the general election - actually knew ... then he was party to something which was unacceptable and unlawful," Osborne told British Broadcasting Corp. radio.
"I can't possibly see how this man stays in post beyond today," he said.
Labour's General-Secretary Peter Watt quit on Monday, acknowledging he knew the donor was channeling funding through associates to dodge transparency rules, but had not informed authorities.
Brown called the practice unacceptable, pledging to pay back the funds and hold a review into party funding.