Labour Cabinet minister Peter Hain said further donations to his deputy leadership campaign "were not registered as they should have been".
Work and Pensions Secretary Mr Hain said he had informed the Electoral Commission and was reviewing all donations to his campaign.
Mr Hain last week admitted he had failed to register a £5,000 donation from Labour's chief fundraiser Jon Mendelsohn.
He said the situation was "extremely regrettable and I apologise".
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.
"I am preparing a full declaration to the Electoral Commission.
"As soon as I discovered on Thursday that the donation from Jon Mendelsohn had not been registered I took immediate steps to do so and made this public.
"I am now doing the same about these further donations.
"This is extremely regrettable and I apologise."
