Cabinet minister Peter Hain has said more donations to his Labour deputy leadership campaign "were not registered as they should have been".
Mr Hain was one of six deputy leadership contenders
Mr Hain has already admitted one £5,000 donation was not registered, blaming an "administrative error".
He is reviewing all donations and apologised for the "extremely regrettable" events.
Harriet Harman has already pledged to pay back a £5,000 donation to her, successful, deputy leadership campaign.
She said she did not know the money, donated under the name Janet Kidd, was actually from property developer David Abrahams who is at the centre of a wider row over donations to the Labour Party.
Police are investigating more than £650,000 he donated to the party over four years, under the names of four associates.
BBC political correspondent James Landale said he understood the donations to Mr Hain's campaign had not come from Mr Abrahams - but had not been declared as they should have been.
In a statement, Work and Pensions and Welsh Secretary Mr Hain said: "In light of recent events I have undertaken a review of all donations to my deputy leadership campaign.
"This afternoon I have been to see the Electoral Commission to inform them that further donations to my campaign were not registered as they should have been."
He said he was preparing "a full declaration" to the Electoral Commission, adding: "This is extremely regrettable and I apologise."
But Liberal Democrat frontbencher Chris Huhne told the BBC: "It is astonishing that Cabinet ministers seem to think that the laws they introduce do not apply to them.
"This appears to be the sort of sloppiness, or worse, that is spreading like a contagious disease through Labour's senior ranks."
Meanwhile the Conservatives have decided to force a Commons debate on party funding on Tuesday. They had intended to use their "opposition day debate" slot to discuss the politicisation of the civil service.
Mr Hain was one of six contenders for Labour's deputy leadership, which was eventually won by Ms Harman. He raised around £77,000 towards the campaign, according to the Electoral Commission.
Last week he said he had discovered a £5,000 gift from Mr Mendelsohn, Labour's chief fundraiser, had not been registered with the Electoral Commission.
The row over donations has also spread to Labour's leader in Scotland, Wendy Alexander, whose campaign team has admitted accepting an illegal donation from a Jersey-based businessman.
Prime Minister Gordon Brown has said he is ready to assist the police investigation into the donations made by Mr Abrahams.
By law, anyone donating more than £5,000 must be identified and relevant details about them disclosed.
Sir Alistair Graham, the former chairman of the Committee on Standards in Public Life, told the BBC: "We do expect a higher degree of rigour in these matters, particularly when this was the party that actually introduced the legislation to produce this transparency.
"And I think really the public will take a very dim view of these sorts of lapses, even when, you know, a full and generous apology is made after the event."
Earlier Mr Brown said he wanted to move "quickly" to reform party funding in the wake of the row over Mr Abrahams' donations.
But the Conservatives have accused him of trying to shift attention from the row, which they say boiled down to top Labour officials professing ignorance of "basic bits of the law".