Gavin Williamson was fired as defence secretary after Theresa May was informed that he had attacked her in private, saying that her diabetes made her unfit to be prime minister.

May became frustrated with Williamson’s behaviour after hearing that he told fellow Tories that her health meant she should not continue in the job — claims that Williamson rejects as categorically untrue.

The warnings were delivered in the weeks before the former defence secretary was sacked amid claims that he leaked details of a National Security Council meeting last week about the Chinese firm Huawei.

Sources at the top of government and the Conservative Party say slurs about the prime minister’s health were overheard by a senior party official, who reported the former defence secretary’s