Sir Michael Fallon has resigned as Defence Secretary following allegations of inappropriate behaviour.
In a statement announcing his resignation, Sir Michael said: "In recent days allegations have been made about MPs' conduct, including my own.
"Many of these have been false, but I realise that in the past I have fallen below the high standards that we require of the armed forces which I have the privilege to represent.
"I have therefore reflected on my position in Government and I am resigning as Defence Secretary.
"It has been a privilege to have served as Defence Secretary for the last three and half years, and I have nothing but admiration for the professionalism, bravery and service of those men and women who keep us safe."
Responding to Sir Michael's resignation letter, Prime Minister Theresa May 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."
The shock announcement comes after Sir Michael was caught up in the sexual harassment scandal sweeping Westminster. It emerged that he had repeatedly put his hand on a journalist's knee during a party conference dinner in 2002.
However, radio host Julia Hartley-Brewer said she had not thought anything more of the incident as "anything but mildly amusing".
Reacting to the news of the resignation, Julia Hartley-Brewer tweeted:
"Bloody hell. Sir Michael Fallon has just resigned as Defence Secretary."
She added that "I doubt my knee was the reason" for his resignation.
PM Theresa May is expected to appoint a replacement on Thursday.
Both Downing Street and the MoD have confirmed the resignation.