A humilated Sir Michael Fallon has quit as Defence Secretary - the first major political scalp of the sex scandal sweeping Westminster.

It came just 24 hours after Theresa May, the Prime Minister, refused to say whether she had confidence in the Scot after it emerged he placed his hand on a former Sunday Express journalist's knee.

The incident happened 15 years ago at a dinner, attended by married Sir Michael and Julia Hartley Brewer, who was political editor at the Sunday tabloid at the time.

It was described as a 'fleeting' incident, but in the context of the scandal sweeping the corridors of power at Westminster made front page headlines in The Sun.

Ms Hartley Brewer - now a presenter and political pundit with an LBC Radio phone-in - said it was nothing more than 'mildly amusing.'

Perth-born Sir Michael has been a key figure in the cabinet for several years and is married to HR professional Elizabeth Payne, and the couple have two sons.

In his resignation letter to Mrs May, Sir Michael wrote: “A number of allegations have surfaced about MP’s in recent days including some about my previous conduct.

“Many of these have been false, but I accept that in the past I have fallen below that we require of the Armed Forces that I have the honour to represent.

“I have reflected on my position and am therefore resigning as Defence Secretary.”

He said it had been a privilege to serve under four prime ministers.

In her reply, the Prime Minister said "I appreciate the characteristically serious manner in which you have considered your position, and the particular example you wish to set to servicemen and women and others."

It is likely to increase pressure on First Secretary and Mrs May’s deputy, Damian Green after the Times reported that he sent suggestive text to an academic he had previously met for drinks. He has denied doing anything wrong.

Ms Hartley-Brewer reacted with shock to the announcement, writing on Twitter: "Bloody hell. Sir Michael Fallon has just resigned as Defence Secretary."

She added that "I doubt my knee was the reason" for his resignation.