Ruth Kelly’s resignation may have surprised delegates at Labour’s conference but she did not hide her feelings when asked about rumours earlier in the week.

Asked by this newspaper if she planned to resign, Ms Kelly, 40, simply smiled and offered no comment. In retrospect, her calm and serene response said it all.

Ms Kelly says that she is leaving to spend more time with her family. She was, however, growing increasingly unhappy with Gordon Brown’s leadership, despite her assurances yesterday that he was a “towering figure”.

It was widely believed that she was going to be sacked as part of a reshuffle in the coming weeks and with a majority of just over 2,000, her seat is vulnerable to a Conservative revival. In short, she may have jumped before she was pushed.

A long-standing Blairite, she is a close friend of both James Purnell, the Work and Pensions Secretary, and David Miliband, the Foreign Secretary, with whom she had a brief relationship in her twenties.

Her stance against the Embryology Bill, allowing the use of hybrid human-animal embryos, also helped to sour an already wilting relationship with the Prime Minister. Once she returns to the back benches she will be free to air personal views on subjects such as stem-cell research.

Her decision brings to an end a whirlwind career. Tony Blair made her his youngest Cabinet minister, at 36, and she is the only woman to give birth to four children while an MP.

The “Supermum”, as she was nicknamed, moved quickly through government ranks. After taking the marginal seat of Bolton West in 1997, she was swiftly made a parliamentary aide to Nick Brown, then the Agriculture Minister, getting her first experience of crisis management in the foot-and-mouth disease outbreak. She joined Mr Brown at the Treasury, where she was appointed a junior minister in 2001, moved briefly to the Cabinet Office working on policy development, and then made the big leap to the Cabinet, as Education Secretary, in 2004.

The transition was not smooth and her two-year tenure was unhappy and devoid of great achievement. From the start, her appointment was questioned because of her links to Opus Dei, the secretive, conservative Catholic movement that inspired the film The Da Vinci Code.

She came under fire again when it emerged that some sex offenders were working with children in schools, and she faced personal pressure last year for her decision to send one of her children to a £15,000-a-year private school. Later, as Communities Secretary, she was at the helm for the botched introduction of home information packs.

As her career rocketed, she underwent a makeover, transforming herself from what some uncharitably called an ugly duckling. Her once cropped student hairstyle has been replaced with blonde flowing locks and her face is always impeccably made up, even yesterday, despite a 3.30am rewrite of her conference speech.

Ms Kelly was born in Limavady, Northern Ireland, and educated at Sutton High School, Westminster School, The Queen’s College, Oxford, and the London School of Economics. In 1996 she married Derek Gadd, a local government officer, who changed his job to help her career.

Former colleagues at The Guardian, where Ms Kelly had her first job as an economics reporter, recall how she went to Mass every day, even giving up her lunchtime if she missed church on her way to work.

Ms Kelly has always taken a rigorous approach to getting the right balance between her work and family life, thanks partly to a husband who changed jobs so that she could pursue her political ambitions.

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