Peter Hain now faces a damaging "sleaze" investigation after an official complaint was made following failure to register donations.

John Lyon, the office of the Parliamentary Standards Commissioner, confirmed he had received a formal complaint concerning the conduct of the Work and Pensions Secretary.

No details of the complaint were released, but it follows Mr Hain's admission yesterday that he failed to declare more than £100,000 in donations to his Labour Party deputy leadership campaign.

Labour in Scotland is still waiting for the result of the Electoral Commission investigation into illegal donations made in Wendy Alexander's leadership campaign.

In Hain's case, Mr Lyon will now have to decide whether an investigation into the complaint is warranted.

If he does launch a full inquiry, he will report ultimately to the powerful Standards and Privileges Committee which can issue a rebuke or, in the most serious cases, recommend suspension from the House.

Mr Hain last night confirmed that he had failed to declare 17 donations totalling £103,156.75 - more than half the money received by his failed deputy leadership campaign.

In a statement, he blamed the pressures of his then job as Northern Ireland Secretary for his failure to ensure that the Electoral Commission was properly informed of the donations.

However the scale of the under-reporting caused astonishment among MPs and triggered calls for him to consider his position.

Shadow work and pensions secretary Chris Grayling said that it had shown "breathtaking incompetence".

However allies today sought to rally round the beleaguered minister, insisting that it was a "perfectly innocent oversight".

Labour MP Martin Linton, who was part of Mr Hain's campaign team, said it was not "a big deal" and that such things could "happen very easily in politics".

"Anyone who has worked with Peter Hain... would know that it could only be what he says it is - a perfectly innocent oversight - and he is very sorry for it," he told the BBC Radio 4 Today programme.

He said the money involved was "a fairly modest amount", comparing it with the 260 million dollars (£133m)spent by US President George Bush on the Republican primary elections in 2004.

"It's easy to make £100,000 sound a lot of money and, of course, for an individual to have it is - I would love to have that kind of money myself.

"But you have got to look at the size of the task: if you are trying to campaign for the votes of three or four million people, even if you just wrote a letter to all of them it would cost half a million pounds."

He added: "You've got to remember with political campaigns that you are all volunteers, you are all unpaid, you are all doing different jobs and you do your job and you hope everybody else does what their job is.

"Peter Hain was the Secretary of State for Northern Ireland, he was in the middle of the Stormont handover. He was certainly appearing at the office from time to time but he was concentrating on his day job.

"So these things can sadly happen very easily in politics."