Mr Hain has apologised for the undeclared donation
Work and Pensions Secretary Peter Hain failed to declare £103,000 in donations to his Labour deputy leadership bid, his spokesman has confirmed.
Mr Hain, who is also Welsh Secretary, came fifth out of six in the race to succeed John Prescott last summer.
He said he should have given "higher personal priority" to the campaign, but he had put his government job first.
His Tory shadow Chris Grayling said the "extraordinary" revelations suggested a "complete disregard for the rules".
All "regulated donees" are expected to report any donation over £1,000 to the Electoral Commission, within 30 days of accepting it.
Mr Hain expressed regret that donations were not reported on time, but said they were all from people legally entitled to donate.
Earlier he 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, he said: "I understand that people will ask how I could have allowed this number of donations to go undeclared at the time.
"The fact is that during this period, I gave my campaign for office within the Labour Party second priority to my government responsibilities.
"I reasonably believed that the arrangements in place for my deputy leader campaign would be sufficient to ensure compliance with reporting requirements, but as it transpired, due to administrative failings this was not the case after early May."
He went on to say that fundraising had to continue after the deputy leadership campaign ended - owing to "unpaid invoices coming to light during the summer and autumn".
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.
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".
And for the Tories, Chris Grayling said: "Failing to declare £100,000 of donations, which is the equivalent of the maximum spending limit on a Conservative leadership campaign, demonstrates breathtaking incompetence.
"Mr Hain is running two government departments. How on earth did he manage to get this so dramatically wrong?"
Lib Dem frontbencher Norman Baker said: "This is a lot of money not to notice and not to report on time, even if you are busy working as a Cabinet minister.
"It looks simply like an oversight, but obviously the Electoral Commission will have to satisfy itself on that."
Questions about Peter Hain's deputy leadership donations first emerged in November - 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.
On 29 November Mr Hain admitted failing to register a £5,000 donation to the Electoral Commission - something he blamed on an "administrative error" which he wanted to declare "in light of recent events".
Days later he said there were other donations that had not been properly declared.
After weeks of work going back through the paperwork, Mr Hain submitted a full list of donations to the Electoral Commission on Thursday.
Meanwhile the two men who ran his deputy leadership campaign have been arguing over who was to blame.
On Wednesday, former Hain aide Phil Taylor, who ran the campaign in its earlier stages, said all donations had been declared when he was in charge.
His successor, Steve Morgan, has said he was brought into the Neath MP's campaign "to bring order to the chaos" left by Mr Taylor.