The pressure on Peter Hain has intensified after an official complaint was made to the Westminster "sleaze" watchdog.

The office of the Parliamentary Standards Commissioner John Lyon confirmed that he had received a formal complaint concerning the conduct of the Work and Pensions Secretary, who is also Welsh Secretary and MP for Neath.

The complaint against Mr Hain was made by the Tory MP for Monmouth, David Davies, the MP's office said. No further details have been released, but it follows Mr Hain's admission on Thursday that he failed to declare more than £100,000 in donations to his Labour Party deputy leadership campaign.

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 confirmed on Thursday night 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.

But 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".

Allies 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