Cabinet minister Peter Hain is expected to face more questions about why he failed to declare £103,000 in donations to his Labour deputy leadership bid.
Plaid Cymru has called for Peter Hain's resignation after fresh revelations about the funding of his Labour deputy leadership bid.
The Work and Pensions Secretary did not register 17 donations from firms, individuals and trade unions with the Electoral Commission as required.
Elfyn Llwyd, Plaid's parliamentary leader, said the work and pensions secretary's position was "untenable".
He told the Guardian newspaper his campaign team could not explain why they had not been declared.
The call comes after it emerged a trustee of a think tank which channelled £25,000 into Mr Hain's bid was also the campaign's treasurer.
The Conservatives have described the revelations as "extraordinary".
The cash in the form of five donations was not declared to the authorities.
Donation rules
Mr Hain, who is also Welsh Secretary and Neath MP, came fifth out of six in the race to succeed John Prescott last summer.
He declared donations of £82,000, but at the end of November he revealed that he had failed to register a £5,000 donation from Jon Mendelsohn, Labour's chief fundraiser.
Then at the beginning of December, Mr Hain said other donations to his campaign "were not registered as they should have been", including a £1,300 campaign dinner.
I was not watching my back and unfortunately it appears others were not either Peter HainWork and Pensions Secretary
All "regulated donees" are expected to report any donation over £1,000 to the Electoral Commission, within 30 days of accepting it.
On Thursday, Mr Hain said he should have given "higher personal priority" to the campaign, but that he had put his government job first.
He later told the Guardian: "I was not watching my back and unfortunately it appears others were not either.
"I just want to make it clear that when mistakes have occurred in the past I have never dumped on assistants or civil servants. I will not start doing so now. I take full responsibility."
'Second priority'
He told the paper that everything was in order until May, but that "unaccountably" no donations had been registered since then until he discovered the problem.
"...and frankly no one, including me, has been able to explain why it suddenly stopped," he added.
Mr Hain has now passed full details of the £103,156.75 in donations to the Electoral Commission, which is due to report back next week.
In a statement earlier, he said he understand people would ask how he could have allowed this number of donations to go undeclared.
"The fact is that during this period, I gave my campaign for office within the Labour Party second priority to my government responsibilities."
He said he had only become aware there was a problem on 29 November 2007 and had "immediately" taken steps to inform the Electoral Commission.
Transparancy 'important'
The BBC's political editor Nick Robinson said it was clear Mr Hain's team had spent so much money that even after the contest ended they had to raise £70,000 for a campaign he had already lost.
But although it was a breach of electoral law, the Electoral Commission had no powers to penalise him - although he may be found to have breached House of Commons rules, he added.
Earlier, the new Parliamentary Standards watchdog Sir Christopher Kelly told MPs his committee "would be concerned that even now not everyone appears to have understood the importance of being absolutely transparent about political donations".
Mr Hain's Tory shadow Chris Grayling said failing to declare £100,000 of donations demonstrated "breathtaking incompetence".
And Lib Dem frontbencher Norman Baker said it was "a lot of money not to notice and not to report on time, even if you are busy working as a Cabinet minister".
Questions about Mr Hain's deputy leadership donations emerged amid a separate donations row engulfing the Labour Party.
The police began to investigate more than £650,000 of proxy donations made to the party by a property developer under other people's names.
Other problems then emerged with donations to Harriet Harman's deputy leadership bid and Wendy Alexander's Scottish Labour leader campaign.
