Chief Whip Geoff Hoon has said the leak of the news that transport secretary Ruth Kelly was to resign was a "serious mistake".
Mr Hoon admitted that while the revelation was unlikely to have been deliberate, it was an unnecessary blow to the government.
He also claimed the emergence of Ms Kelly's departure from the Cabinet had detracted from prime minister Gordon Brown's impressive speech at the Labour party conference the previous day.
"I don't think anyone could have been happy about that situation, least of all Ruth Kelly. I thought she handled it with commendable dignity," Mr Hoon told Sky News.
"I thought that whoever was responsible for that particular piece of briefing acted irresponsibly and not in the best interests of the government. Therefore I certainly regret it.
"I doubt actually, given the circumstances, that it was done deliberately. I think this kind of late night gossiping is a feature, unfortunately, of party conferences."
He added: "What I do say is that whoever was responsible did make a serious mistake and that has not helped the government."
Ms Kelly stepped down in order to spend more time with her family.
She will remain in the Cabinet until Mr Brown imposes a reshuffle, which could happen in the next week.