The move came as details of a four-year affair with Petronella Wyatt, the daughter of the late Lord Wyatt, were revealed by her mother.

Johnson, 40, who became a household name through his appearances on the television show Have I Got News for You, had previously denied any romance. He said last night that he felt as if he had been hit by “an express train” over the allegations.

Speaking from the mansion in St John’s Wood, north London, where she lives with her daughter, Lady Verushka Wyatt disclosed that her daughter had an abortion last month as a result of an affair with Johnson, who is married with four children.

Last night she told The Sunday Times: “In the beginning, the reason she went out with him was because he said he was going to marry her. (Otherwise)Petronella never went out with a married man.”

The pregnancy was unplanned and occurred when Johnson sought to rekindle the relationship. “It was just one of those things which happened, he came along and told her he still loved her. It was one of those accidents which happened which nobody really wanted to. That is the truth.”

It brings to an end Johnson’s fledgling career in frontline politics, although he will remain on the Conservative back benches as the MP for Michael Heseltine’s old seat of Henley. The 40-year-old’s wit, intellect and affable nature had led to him even being talked about as a future Tory leader.

The final straw for Howard came when it was clear that Johnson, whose wife Marina is the daughter of Charles Wheeler, the veteran BBC correspondent, had lied over the affair with Wyatt, 35.

When stories appeared about the relationship. Johnson had dismissed them as “an inverted pyramid of piffle”.

Howard said the sacking was because Johnson had lied over the affair. It had nothing to do with morality.

Johnson last night refused to go into any of the details of his private life, but he added: “It’s difficult and much of what I have heard is very peculiar.”

His first marriage to Allegra Mostyn-Owen, a socialite, lasted only a year. The MP — “Bozzer” to parliamentary chums — and Wyatt — “Petsy” to her friends — started their romance when he was appointed editor of The Spectator where she was first deputy editor and then a columnist.

Friends of Wyatt say she believed him when he told her he would leave his wife for her.

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