Sir Michael Fallon has resigned as Defence Secretary, a Downing Street spokesman has announced.
The Sevenoaks MP wrote in a letter to Theresa May: "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 represent.
"I have reflected on my position and I am therefore resigning as Defence Secretary."
The PM responded, saying: "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."
She praised his "long and distinguished" career in Parliament and added: "On behalf of all those you have helped throughout your time in Government, I thank you for your service."
It comes days after Sir Michael apologised for putting his hand on a journalist's knee.