Gordon Brown admitted last night that Peter Hain's campaign was guilty of "incompetence" over undeclared donations to his deputy leadership campaign but insisted he would be cleared of wrongdoing.
The Work and Pensions Secretary has recently admitted failing to declare £103,000 in donations to his failed campaign within the time limit.
The Prime Minister told ITV's News at Ten that Mr Hain "has confessed to his mistakes and he has apologised in the most profuse terms for what has gone wrong". He added: "It was a mistake that was made, it was an incompetence that he has readily admitted to.
"This now goes before the Standards Committee in the House of Commons and before the Electoral Commission and I believe that they will understand that this was a failure but there was no corruption involved, no illegal donation made, and I hope that they will be able to accept his apology."
Last night Mr Hain's Cabinet colleague Ed Balls, a confidant of the Prime Minister, said it was important for him to "answer all questions" about the donations.
Mr Hain faces a potentially torrid Question Time in the House of Commons today immediately before Mr Brown is questioned by the Conservative leader, David Cameron.
Last night Mr Hain's Cabinet colleague Ed Balls, a confidant of the Prime Minister, said it was important that the Government maintained "the highest standards in public life", and that it was important for Mr Hain to "answer all questions" about the donations.
The Children's Secretary told BBC Radio 4's World at One programme: "Of course it is important that Peter answers all the questions in the inquiry, we have the kind of scrutiny which you should have, but at the same time I think it is important that we get on with the job of delivering for the British people."
The Electoral Commission, the official elections watchdog, is considering referring the case to the police. The Parliamentary Standards Commissioner, John Lyon, has also launched an inquiry into whether Mr Hain broke House of Commons rules by failing to declare the donations in the Register of Members' Interests.
Mr Hain's undeclared donations included £14,500 from the GMB union, which has campaigned for his department to back down over the closure of Remploy factories which offer jobs to thousands of disabled people.
In September Mr Hain used his speech to the Labour party conference to declare that no Remploy factories would close without ministerial approval, and pledged there would be "no compulsory redundancies for Remploy's disabled workers. Salaries guaranteed. Full final salary pensions protected." In November Mr Hain announced that 28 Remploy factories would be closed or merged.
The GMB has denied attempting to use its contributions – which paid for a leaflet campaign and national newspaper advertising – to boost its campaign for the Remploy workers.
A spokesman insisted that the decision to support Mr Hain's campaign was taken in June after the party's conference voted to endorse his candidacy and before Mr Hain was given the job of Work and Pensions Secretary.