Ruth Kelly’s announcement that she will quit Cabinet at the reshuffle is a massive blow to Gordon Brown because of its timing.

The Prime Minister's conference speech had been well received and appeared to have given him breathing space. It appears that Ms Kelly had told Mr Brown some time ago that she wanted to leave office at the time of the next reshuffle and she insisted today that she was leaving purely to spend more time with her family.

However she is one of a group of ministers who were known to be unhappy with the direction of Mr Brown’s leadership. She was widely seen as one of the ministers most likely to be dropped in any revamp and the assumption among Labour MPs this morning was that she had decided to jump before being pushed.

Ms Kelly was once seen as being closer to Mr Brown than to Tony Blair, who brought her into government to work under Mr Brown at the Treasury. But relations have deteriorated in recent years.

News of Ms Kelly’s intention emerged late last night and was confirmed by Downing Street sources in the early hours. However the precise reason for the disastrous timing of the announcement is unknown. It is assumed that fellow ministers who knew of Ms Kelly’s predicament revealed her intentions.

Times Recommends

• Margot Fonteyn: In her footsteps
• Forget beautiful people, let's have wonky Brits
• Fundamentalist brew: faith and power