Sir Michael Fallon’s decision to resign as Defence Secretary was triggered by a claim that he had carried out a sexual assault as well as engaging in inappropriate behaviour, it has now been claimed.

No 10 was presented with the allegation just hours before his resignation was announced on Wednesday evening.

The revelation that the nature of the allegations against Sir Michael now includes sexual assault is bound to deepen the crisis at Westminster, which has seen claims laid against a growing number of MPs.

Sir Michael had apologised on Monday night for having repeatedly put his and on the knee of journalist and broadcaster Julia Hartley-Brewer, saying his personal conduct “had fallen below the high standard” he should have displayed as Defence Secretary.

Friends at the time said he was not stepping down because he feared new allegations may emerge.

It then emerged that Andrea Leadsom, leader of the Commons, had accused Sir Michael of making lewd remarks about her and other women and of engaging in unwanted physical contact.

But now Anna Soubry, the former Conservative minister for business, has said the allegations against the former Cabinet minister go still further.

The former barrister said: “Michael Fallon had to resign because of his behaviour towards women. One person with great courage made a complaint to No 10 of sexual assault.