He said: "In the past I have fallen below the high standards that we require of the Armed Forces"
Michael Fallon has resigned as the UK's defence secretary, a Downing Street spokesman has announced.
In a letter to British Prime Minister Theresa May, the MP wrote: "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."
Mrs May responded: "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 Mr Fallon apologised for putting his hand on a journalist's knee.
The incident involved commentator Julia Hartley-Brewer, and happened at a Conservative party conference dinner in 2002.
A spokesman for Mr Fallon said when the news emerged on Monday: "He had apologised when the incident happened 15 years ago and both Julia and he now considered the matter closed."
Ms Hartley-Brewer said: "I have had no issues since with the man in question and do not regard the incident as anything but mildly amusing."