George Osborne has denied breaching his own party's sleaze rules by accepting thousands of pounds for appearing at an event with business chiefs.
The shadow chancellor received up to £10,000 for speaking to the Institute of Directors' annual conference in Jersey.
But Tory guidelines state: "Shadow Ministers should not solicit fees for broadcasting, speaking engagements and articles. Fees should not be accepted if the subject matter relates directly to your Shadow Ministerial responsibilities."
In the Register of Members' Interests, Mr Osborne lists last November's engagement as a "speech".
However, Mr Osborne has insisted it had actually been a wide-ranging question and answer session with BBC presenter Andrew Neil, and dismissed claims he had done something wrong.
"I registered it in the proper way," he told BBC1's Andrew Marr programme. "It is not against Conservative rules, it is not against Parliamentary rules because I properly registered it."
Mr Osborne said the "more interesting question" was how to remove the "whiff of corruption" from politics, accusing Gordon Brown of blocking moves to reform Commons allowances last week.
Labour MP John Mann called for the speaking fee to be repaid. "David Cameron cannot lecture others if he isn't even enforcing his own party rules," Mr Mann said. "He should order all of this money to be paid back."
Other Tory frontbenchers were also facing claims of a conflict of interest.
Shadow schools secretary Michael Gove received a donation from the director of an independent boarding school. Meanwhile, his deputy Tim Loughton earns up to £35,00 a year as director of a company that provides camera equipment for schools.
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