Sir Michael Fallon has resigned as Britain's Defence Secretary in the wake of allegations about sexually inappropriate behaviour after admitting that his past conduct may have "fallen below" the high standards expected of him.
He earlier this week admitted that he had put his hand on the knee of a radio presenter at a dinner 15 years ago.
In a letter to the Prime Minister, Fallon said: "A number of allegations have surfaced about MPs in recent days, including some about my previous conduct.
"Many of these have been false but I accept in the past I have fallen below the high standards that we require of the Armed Forces that I have the honour to represent.
"I have reflected on my position and I am therefore resigning as Defence Secretary."
He added: "I have been fortunate in the opportunity to serve in the Governments of four Prime Ministers, and to have played a part in education reform, including setting up Ofsted, the reduction of red tape for business and the attraction of new investment into energy generation."
Fallon admitted the incident had taken place and said he had apologised to broadcaster Julia Hartley-Brewer for his behaviour at the time.
Hartley-Brewer previously disclosed the incident although she did not name Fallon. She said that at the time she had threatened to punch him in the nose if he did it again.
Unlike other incidents, the Prime Minister did not refer Fallon for a Cabinet investigation.
The Telegraph understands that there will not be a reshuffle but that Fallon will be directly replaced.
A close ally of Fallon said earlier this week: "Julia's a good friend of Michael's. He overstepped the mark when he put his hand on her knee. She made clear it was unwelcome and he rightly apologised 15 years ago."
It comes amid a growing sexual harassment scandal at Westminster, with some 40 Conservative MPs being named in a so-called 'dirty dossier' compiled by Tory staff of politicians against whom accusations have been made.
Theresa May has come under pressure from her own party to investigate alleged sexual harassment.
Hartley-Brewer tweeted a statement saying: "I have spoken previously about a Cabinet Minister who repeatedly put his hand on my knee during a party conference dinner.
"I calmly and politely explained to him that if he did it again I would 'punch him in the face'.
"He withdrew his hand and that was the end of the matter.
"I have had no issues since with the man in question and do not regard the incident as anything but mildly amusing, which is why I have declined to name him."