Kent frontbencher Sir Michael Fallon has resigned.
The Sevenoaks MP and Defence Secretary earlier admitted touching a journalist’s knee, as allegations of alleged misbehaviour among MPs surface, according to a national newspaper.
In a resignation statement, he 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 that in the past I have fallen below the high standards that we require of the Armed Forces that I have the honour to respresent.
“I have reflected on my position and I am therefore resigning as Defence Secretary.
“It has been a privilege to have led Defence over the past three and a half years.
“We are close to defeating Daesh terrorism in Iraq and Syria; we have taken a leadership role in NATO; and we have secured a defence budget that will grow ahead of inflation each year.
“I have the utmost admiration for the professionalism, bravery, and service of the men and women who keep up safe.
“I have been fortunate in the opportunity to serve in the governments of our 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.
“I shall continue to support the government and work hard for my Sevenoaks constituents.”
Paul Francis, Political Editor on a dramatic day in politics with Kent MPs at the centre of events:
A day that started with the spotlight fixed firmly on one Kent MP has ended with the focus switched to another.
Amid a swirl of allegations surrounding the behaviour of MPs, the shock resignation of Sir Michael Fallon as defence secretary is a measure of just how serious the issue of harassment and inappropriate behaviour has become for Parliament.
Just a few days ago Sir Michael publicly acknowledged he was the politician a journalist claimed had repeatedly stroked her knees at a party function.
Sir Michael said he had since reflected that his behaviour had fallen short of what the public might expect but did not go into details, saying what might have been acceptable years ago was no longer deemed to be so.
His decision to quit comes on a day that saw another Kent MP under scrutiny. Ashford MP Damian Green, effectively Mrs May's deputy, is now the subject of a cabinet inquiry into allegations - which he denies - of inappropriate conduct.
Whatever the outcome of that is, Theresa May has lost a key ally and - up until now - a reliable cheerleader. It was not the one she might have expected and has overshadowed her announcement of a plan for "proper processes" to be introduced to report claims of harassment by MPs.
She will be reluctant to lose another.