Peter Hain's disclosure is an embarrassment for Brown, who is trying to make a fresh start after his first six months as prime minister were overshadowed by a party funding scandal and government blunders that left the Labour Party trailing in opinion polls.
Labour's general secretary resigned and police were called in to investigate last year after it emerged property developer David Abrahams had given more than 600,000 pounds to the party through go-betweens in apparent violation of electoral law.
Hain said in November he had failed to register with the electoral watchdog a 5,000-pound donation to his unsuccessful campaign last year to become the Labour Party's deputy leader because of an administrative error.
He said on Thursday that his campaign had failed to declare donations totalling more than 103,000 pounds within the time frame required by electoral rules. He said he met the Commission on Thursday and provided full details of the donations.
Hain said he had been concentrating on his ministerial duties, relying on campaign officials to comply with election rules.
"I very much regret that these reports were not made on time. I should have given higher personal priority to the day-to-day administration and organisation of my campaign," he said in a statement.
He said all of the donations had come from people who were legally entitled to donate to his campaign.
The Guardian reported earlier this week that the undeclared funds ran into tens of thousands of pounds and said that some political sources believed the oversight could raise questions over Hain's political future.
