The Secretary of State for Transport has addressed the Labour party conference on its final day, hours after it emerged she is to resign from the government.
Ruth Kelly told delegates it had been “the greatest honour of my life to play a part.”
News of her decision to stand down surprised party activists gathered in Manchester.
Ms Kelly, 40, has been an MP since 1997.
She has four children under 12 and said that she wanted to spend more time with her family.
“This will be my last time addressing you as a member of the Cabinet,” she said.
“I told Gordon before the summer of my decision to leave the Government for family reasons at the next reshuffle.
“This was not a decision I took lightly. I was still in my 20s as Labour was given the honour of leading the country.
“And I have been particularly proud as a minister at the Treasury to be able through our proposals on the Child Trust Fund. At Education, opening up school choice for parents, At Communities recasting our policies on violent extremism and devolving more power to local government.
“In transport, I was delighted it was Labour that secured the deal on Crossrail.
“It’s also been a tremendous privilege to have worked with both Tony Blair and Gordon Brown – two towering figures in the Labour Party, Government and on the world stage.
“But Conference, as well as a front line politician, I am also proud to be a mother and wife. To be able to hold these jobs, I have relied on the support of my husband and family.
“So I ask for your understanding when I say that I now owe it to my children and family to put them first. If I do not, then I know that this is something I will come to regret deeply.
“Conference, while we have not always agreed on every issue, I am enormously proud of all that we have achieved together over the past decade.
“And I am confident that Labour has the values, vision and determination to see this country through difficult times.”
Ms Kelly’s resignation comes ahead of an expected reshuffle which could happen as soon as this weekend.
The Prime Minister told BBC Radio 4’s Today programme: “There are no political issues between Ruth and me.
“Ruth has been an MP all the time that her children have been growing up.
“This is the decision that every parent faces. It is nothing to do with politics. Sometimes we have got to make decisions that are difficult.”
His assertions that there is no difference between himself and Ms Kelly has not scotched rumours that she left because she was unhappy with the direction of the government.
Gay rights activists objected to her appointment to the post of Minister for Equality in 2006.
A member of controversial Roman Catholic group Opus Dei, she had never voted in favour of gay rights.
She was heavily criticised for delaying the implementation of the Sexual Orientation Regulations.
Ms Kelly has refused to say whether or not she personally regarded homosexuality as sinful.
Earlier this year she was one of three Roman Catholic Cabinet ministers who voted against equal access to fertility treatment for lesbians and gay men.