Theresa May's Government was thrown into crisis on Wednesday night after the Defence Secretary Michael Fallon became the first minister to resign over the sex scandal engulfing Parliament.

Sir Michael Fallon said that his conduct may have fallen below the standards now expected of those in public life. He did not detail his misdemeanours but said that the “culture has changed over the years”.

The resignation came less than 24 hours after Downing Street announced a Cabinet Office inquiry into whether another senior minister – Damian Green – had acted inappropriately towards a younger woman.

Several other senior ministers have been named in a dossier circulating in Westminster that makes unverified accusations about sexual conduct.

A spokesman for the Prime Minister said earlier this week that any unwanted sexual behaviour is “completely unacceptable in any walk of life”.

The Conservatives now fear that the issue will undermine the Government in the same way that sleaze came to destroy the Major administration of the Nineties.

The Defence Secretary sent his letter of resignation to the Prime Minister at 7.30pm after admitting that he had touched the knee of a female journalist at a conference dinner 15 years ago.