Sir Michael Fallon's resignation follows days of speculation about the sexual harassment scandal sweeping Westminster.
Here is how events unfolded:
- Reports emerge that female researchers and aides in Parliament were using a WhatsApp group to share information about alleged sexual harassment and abuse at Westminster.
- Downing Street describes the allegations as "deeply concerning".
- Prime Minister Theresa May urges anyone with information to contact the authorities.
- Labour Leader Jeremy Corbyn says a "warped and degrading culture" is thriving in Westminster and that MPs who engage in the abuse and sexual harassment of women must be held accountable for their actions.
- Four male MPs, including a minister, are accused of harassing or propositioning young women inappropriately.
- International Trade Minister Mark Garnier admits asking his secretary to buy sex toys for him and calling her "sugar tits".
- The Cabinet Office launches investigation into whether Mr Garnier he broke the Ministerial Code.
- Mrs May calls for reform of the House of Commons' "toothless" disciplinary procedures.
- House of Commons Speaker John Bercow describes allegations of a "culture of sexual harassment" at Westminster between MPs and their staff as "disturbing".
- Senior parliamentary authorities meet to discuss a new independent helpline to deal with complaints of sexual harassment and abuse at Westminster.
- Defence Secretary Sir Michael Fallon apologises for putting his hand on journalist Julia Hartley-Brewer's knee 15 years ago.
- Mrs May's spokesman says she "has confidence in her Government and her ministers".
- Prominent Labour activist Bex Bailey says she was raped at a party event in 2011 and felt discouraged to report it as it could have damaged her career.
- Labour says it takes the allegations by Ms Bailey "extremely seriously" and launches an independent inquiry into the claims.
- An anonymous woman claims she was sexually assaulted by an MP on a foreign work trip last year and that her allegations were not taken seriously.
- Mrs May asks Whitehall's top mandarin to investigate allegations that her effective deputy, Damian Green, made inappropriate advances towards female activist Kate Maltby in 2015.
- Justice minister Dominic Raab and aid minister Rory Stewart deny any wrongdoing after they were both named on an unverified list of sexual misconduct allegations in Westminster.
- The Prime Minister invites leaders of Westminster's political parties to a crisis meeting to discuss plans for tackling sexual abuse and harassment.
- Sir Michael resigns as Defence Secretary and acknowledges that "in the past I have fallen below the high standards we require of the armed forces".
