Former Cabinet minister Peter Hain was guilty of "serious and substantial" failings in not registering more than £100,000 of donations to his Labour deputy leadership campaign, according to Westminster's sleaze watchdog.
The Commons Standards and Privileges Committee said the scale of the rule breach caused "justified public concern".
But the committee merely ordered the Neath MP to make an apology on the floor of the House after accepting there was no "intention to deceive".
In a statement, Mr Hain said: "The Cabinet Secretary stated that I complied fully with the Ministerial Code, the Crown Prosecution Service (CPS) exonerated me and now the Parliamentary authorities have also accepted that the mistakes I made were honest mistakes.
"I have been asked to repeat my apology on the floor of the Commons, which I am happy to do."
The CPS has already announced it will be taking no criminal action against Mr Hain over the failures.
The Committee's report dismissed the idea that Mr Hain's workload as Work and Pensions Secretary and Wales Secretary was an excuse for the errors.
"This is a case of an experienced Member, a Cabinet Minister at the time, failing in his duty as a Member of Parliament to register donations within the time required by the House," it said.
"We understand that the pressures on Ministers and on front-benchers can be onerous, but we cannot accept - and we are sure that none of them would suggest - that this excuses them from their obligations under the rules of the House."
The report indicated that usually the failures would have attracted a "heavier penalty", but Mr Hain had already lost his job.
"Because of the seriousness and scale of this breach and noting the considerable, justified public concern that it has created, we would ordinarily have been minded to propose a heavier penalty.
"However, we accept that there was no intention to deceive and Mr Hain has already paid a high price for his omissions."
Mr Hain resigned from the Government last January when the Electoral Commission triggered a police investigation into his late declaration of around £130,000 in donations.
Some 19 gifts were handed to his failed Labour deputy leadership bid between May and November 2007, but not listed with the Register of Members' Interests within the four-week time limit.
Standards Commissioner John Lyon identified a change of campaign manager as an "important factor" in the failures.
"Another important factor was the disbandment of the campaign team while donations were still coming in, and before the large debts that were incurred during the campaign had come to light," the committee said.
"A further important factor was Mr Hain's failure to identify any continuing need for registration after it ceased in May 2007, until prompted to do so by one of his donors in November."
It is understood that Mr Hain is likely to make his apology to the House on Monday.
The Commissioner said that, in his evidence, Mr Hain had "noted the priorities and pressures" he was under as Northern Ireland and Wales Secretary until the deputy leadership election finished on June 24, and Work and Pensions and Wales Secretary as part of Gordon Brown's first Cabinet.
However, he had "not sought to avoid his own responsibility for having failed to report all his donations".
Mr Hain's aide, Phil Taylor, was replaced by Steve Morgan in early April. All donations were properly registered until May, when there was a "breakdown within his campaign organisation of the system for notifying him of his obligations to register", according to Mr Lyon.
After last month's CPS announcement that Mr Hain would face no action over the £103,156.75 declared late, he signalled a desire to return to the Cabinet.
But, despite the committee stopping short of recommending his suspension from the Commons, the strength of its criticism is likely to prevent his rehabilitation.