The Transport Secretary Ruth Kelly confirmed today she is to quit Gordon Brown's Cabinet to spend more time with her young family.
The surprise departure of a close ally threatens to take the shine off Mr Brown's well-received speech to Labour's conference in Manchester yesterday, which the Prime Minister is hoping will cement his position as leader and revive the party's performance in the polls.
Ms Kelly will leave the Government at the next reshuffle, which is widely expected to take place at the end of next week, though Mr Brown refused this morning to be drawn on its timing.
Mr Brown said he respected Ms Kelly's decision, which he insisted was "nothing to do with politics".
Announcing her decision in Manchester, Ms Kelly said: "This is purely a decision that has been taken for family reasons."
She said she would continue to support Labour in the future and said she could not think of anyone better than Gordon Brown - whom she described as "a towering figure in the Labour Party" - to lead Britain through the current economic turmoil.
Ms Kelly's decision sparked speculation that the Catholic MP for Bolton West did not want to have to vote with the Government in a forthcoming division on the controversial Human Fertilisation and Embryology Bill.
But the Prime Minister insisted her decision was driven by her desire to spend more time with her four young children. He revealed that she told him of her plan to quit in May.
Mr Brown told GMTV: "She has been an MP for 11 years. All the time that her children were growing up, she was carrying out her job as minister and juggling with work and family life.
"It is a very personal story and I do understand as a father myself that there are difficult decisions we have to take."
Asked about speculation that there were more ministerial resignations in the offing, Mr Brown responded: "That's not true at all."
Ms Kelly said she had told the Prime Minister in the summer that she intended to step down.
"It's not been an easy decision," she said. "The last 10 years have been the proudest of my life."
She told reporters the move had been "purely a family decision".
"This is a decision that has been made absolutely completely for family reasons."
She insisted the timing of the news emerging had been "completely unexpected".
Ms Kelly also stressed that she was a "great supporter of the Labour Party" and it had "transformed" Britain since 1997.
She added that she could not think of "anyone better" than Gordon Brown to represent the UK on the world stage.
Mr Miliband paid tribute to Ms Kelly, who he said had made "a huge contribution" to the Government's work.
Asked if he expected to keep his own job in the expected reshuffle, he said: "I hope so. I'm very happy doing the job I am doing. To represent your country is a fantastic thing to do and I am happy doing that."
He said: "I pay tribute to her. This is her decision and I want to talk to her about her decision.
"She has been an outstanding MP since 1997. I have been in the Cabinet with her for two or three years and she has always been a really good colleague and a decisive minister.
"She's made a huge contribution. I'm sure she will do much more."
Health Secretary Alan Johnson told Sky News: "I am very sorry that Ruth's leaving the Cabinet, but she's got her own personal reasons for doing that and I respect that.
"She's a tremendous political talent and no doubt that political talent will continue to be exercised in whatever capacity she moves into on the backbenches."
The deputy leader and party chairman Harriet Harman will bring the curtain down on conference proceedings with the closing speech to the gathering in Manchester this afternoon.
The Prime Minister, emboldened after his well-received speech to the conference yesterday, will immediately set off for the United Nations in New York to address world poverty and the financial turmoil.
He appeared stronger today after five days in which open criticism of his leadership has been largely absent, although the forthcoming Glenrothes by-election in Scotland remains a significant test.Reuse content