No. 10 blamed for Kelly's exit

The Transport Secretary said she was "shocked" that her decision to step down quietly in the next reshuffle had been shattered by leaks of her plans.

Fellow Cabinet ministers were furious and her friends pointed an accusing finger at No10 officials for "outing" her in an extraordinary 3am briefing at the Labour conference in Manchester.

The bombshell news blotted out the success of the Prime Minister's big speech and disrupted his carefully planned autumn fightback to turn round the polls.

Ministers were bewildered why Downing Street staff appeared to have broken the resignation plan at such a critical moment - with some blaming the misjudgment on paranoia around Mr Brown about a possible leadership challenge.

Amid claims that several members of the Cabinet are ready to march in to demand Mr Brown's removal, Ms Kelly insisted that she was not part of any plot and had made her mind up months ago to quit to spend more time with her family.

The effect of Ms Kelly's departure was to wreck a tour of TV and radio stations this morning by the Prime Minister, who was seeking to capitalise on his successful conference speech but found himself diverted onto questions about his relationship with the Transport Secretary.

While she has never been publicly disloyal, Ms Kelly is said to have had doubts about the Government's direction in recent months.

A close friend of leader-in-waiting David Miliband, she is said to have been unhappy with the Government's political strategy under Mr Brown's leadership.

Last night there were claims from the Tories that Ms Kelly called the Prime Minister's conference speech "terrible".

One of their researchers, Alex Dawson, who is attending the conference as an observer, claimed he heard the remark while walking behind her from the hall.

Ms Kelly flatly denied the allegation and said she thought the speech was "brilliant".

Her name had been linked to rumours that several ministers planned to resign together at the reshuffle and attack Mr Brown's leadership. Some MPs were speculating that Downing Street forced the timing to disrupt any such plot.

But speaking on the BBC, Ms Kelly said: "I have not been part of any plot about the leadership."

Today was her big speech on transport issues - and she publicly praised Mr Brown and Tony Blair as "towering figures". Earlier she told the Standard her only motive was to spend more time with her four young children and husband. "I owe it to my children and family to put them first," she said. "If I do not, then I know that this will be something I will come to deeply regret."

Mr Brown praised her decision, revealing that she told him of her intentions back in May. "She wants to make clear it has nothing to do with politics," said the Prime Minister. "All this speculation that Ruth Kelly's decision is to do with something else, it's actually a pretty straightforward story which I understand as a father."