A major Cabinet reshuffle is imminent after the minister in charge of health, Frank Dobson, resigned to concentrate on his bid to become the new Mayor of London.

Announcing his decision after months of speculation, Mr. Dobson acknowledged the two roles would be too difficult to combine.

The Prime Minister, Tony Blair, will have to carry out a mini-reshuffle of his senior ranks over the next few day to replace the current Secretary of State for Defence, Lord Robertson, who leaves on Thursday to become Secretary General of Nato.

But Mr. Dobson's offer to leave the Cabinet will enable Mr Blair to undertake a wider shake-up than originally expected.