Asked in a BBC interview if the war had been “worth it”, he replied: “I don’t think we can tell yet. I think it is probably up to each person in his own mind to make that decision.”

The biggest mistake had been to allow a security vacuum to develop in Iraq. Not enough resources were put in to control the security situation at the beginning of the war, Sir Jeremy said, and he predicted that the violence would last for “many years”.

The coalition had incorrectly assumed that the operation would be easier than it turned out and that the Iraqis would be able to look after themselves. It had been assumed that Iraqis would be so relieved to be free of Saddam Hussein that they would “march forward into a new Iraq”.

Sir Jeremy said that the British Government would have liked the UN weapons inspectors to have been given another six months, but went along with momentum from the Americans to intervene earlier.

Sir Jeremy’s book about the build-up to the invasion of Iraq, The Cost of War, was vetoed by Jack Straw, the Foreign Secretary. It was due to be published at the beginning of November.

Sir Michael Jay, the Permanent Under-Secretary of State at the Foreign and Commonwealth Office, recently suggested that it could eventually be cleared for publication if some changes were agreed.

The treatment of Sir Jeremy’s book contrasts with that of Sir Christopher Meyer’s recently released memoirs, DC Confidential, which was passed for publication despite criticising several members of the Cabinet.

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