Work and Pensions Secretary Peter Hain was guilty of "an incompetence" over donations during Labour's deputy leader race, the prime minister has said.
Gordon Brown has admitted Peter Hain showed "incompetence"
The Electoral Commission is looking into the late declaration of £103,000 in donations to Mr Hain.
Gordon Brown told ITN Mr Hain made a mistake he had "readily admitted to" but there had been "no corruption".
During Commons clashes he denied that Mr Hain was a "dead man walking" and said he had full confidence in him.
He was speaking at prime minister's questions, which followed straight on from Mr Hain facing MPs for his first scheduled questions session since the row over the donations emerged.
During that session Mr Hain, who is also Welsh Secretary, faced calls to quit his two Cabinet jobs.
He also came under fire over visits to a business in Wales, who it later transpired had given funds to his deputy leadership campaign.
He said he had been "proud to visit" such firms as they were creating jobs in Wales.
But Shadow Welsh Secretary Cheryl Gillan called for the publication of correspondence relating to the visits to ensure Mr Hain was "competent and free of bias".
The prime minister's official spokesman earlier confirmed that contrary to previous reports Mr Hain had briefly apologised to colleagues at Cabinet on Tuesday, for what the spokesman referred to as the "kerfuffle" surrounding his donations.
In his interview with ITN, Mr Brown said he hoped Mr Hain would escape serious punishment.
"It was a mistake that was made. It was an incompetence that he has readily admitted to," he said.
"This now goes before the standards committee in the House of Commons, and before the Electoral Commission.
"And I believe they will understand that this was a failure, that there was no corruption involved, that there was no illegal donation made, and I hope that they will be able to accept his apology."
The Electoral Commission is investigating Mr Hain's failure to inform it of the donations under party funding laws.
Parliamentary Standards Commissioner John Lyon has launched a separate inquiry into whether the Neath MP broke House of Commons rules by failing to declare the gifts in the register of members' interests.
Mr Hain, who came fifth in the six-candidate Labour deputy contest, has insisted he was "open and clear" about the issue and says he will co-operate with any inquiry.
The commission is expected to produce its initial findings by the end of the week, with a full report coming later.
The Liberal Democrats' leader, Nick Clegg said: "When Gordon Brown said he wanted a government of all the talents, no-one realised that incompetence was going to be one of them.
"Whatever the Electoral Commission now decides, Peter Hain's political credibility lies in tatters."
Conservative leader David Cameron has said Mr Hain still has questions to answer, adding: "He can get out there and explain himself or I think he will have to leave the cabinet."
Mr Hain faces questions over the role of a think-tank, the Progressive Policies Forum (PPF), in channelling donations.
The PPF employs no staff and has not published any research since it was set up in December 2006, three months after Mr Hain launched his campaign.
Mr Hain's constituency Labour Party in Neath is due to meet on Wednesday to discuss the affair.