Sir Michael Fallon has quit as the UK's Defence Secretary in the aftermath of being embroiled in the Westminster sex scandal.
In his resignation letter, the long-serving MP said his past behaviour had "fallen short" of what is expected in the military.
Sir Michael is the first ministerial casualty following a series of Westminster sexual harassment allegations, which have emerged in the wake of the Harvey Weinstein revelations.
Earlier this week, he apologised over an incident 15 years ago in which he made unwanted advances to the journalist Julia Hartley-Brewer, placing his hand on her knee.
HuffPost UK understands Sir Michael quit because he couldn't guarantee there would not be another story about contact with a female journalist.
In his resignation statement, 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."
HuffPost UK understands Sir Michael's replacement will be announced on Thursday as the Prime Minister is now forced to reshuffle her Cabinet.
This is his resignation letter:
Hartley-Brewer spoke to Sky News soon after his resignation was announced, and said it would be the "most insane and absurd resignation" if it was a result of what she dubbed 'kneegate'.
She said: "I absolutely didn't expect it - I don't think I can repeat on Sky News the words I actually used at home when I got the first call about this.
"I'm incredibly shocked - if this is over knee-gate and him touching my knee fifteen years ago, me not having any issue with it today, then this is the most insane and absurd resignation of a Cabinet Minister in the history of the world.
"However, I don't think that is the reason. I'm assuming there are more allegations to come, either in a tabloid newspaper tomorrow or sooner than that. There a re rumours swirling around Westminster, none of which I'd be willing to repeat on air for the sake of your lawyers."
The current scandal embroiling Westminster stems from the multiple allegations against Harvey Weintsein that began to emerge last month.
More than 30 women have come forward to accuse the movie mogul of varying degrees of sexual assault, harassment and rape, including Gwyneth Paltrow, Rose McGowan and Cara Delevingne.
Since then similar accusations have been made of prominent figures in a number of industries, including politics.
Labour MP Jared O'Mara was suspended from the Labour Party last week after he was revealed to have posted homophobic and sexist comments online.
And just yesterday a former aide to Liz Kendall, Bex Bailey, revealed that she was raped during a party event - but was discouraged from reporting the assault.