Theresa May is facing a Cabinet reshuffle after Sir Michael Fallon sensationally quit as Defence Secretary.
The shock announcement came after it emerged Sir Michael had repeatedly put his hand on a female journalist's knee at a dinner in 2002.
The 65-year-old apologised for his actions saying his behaviour had "fallen below the high standards required" and acknowledged what might have been acceptable in the past was no longer appropriate.
The resignation leaves the Prime Minister facing a reshuffle and deprives her of one of her most experienced and trusted colleagues.
When allegations of sexual misconduct first began circulating last week, ministers were warned that "serious action" would be taken by Mrs May where necessary.
A list of MPs accused of inappropriate behaviour has reportedly been circulating at Westminster.
Sir Michael's resignation will fuel speculation that other ministers could also be forced out as a result of the scandal.
Cabinet Secretary Sir Jeremy Heywood is looking into claims made against Mrs May's de facto deputy prime minister Damian Green.
It is claimed Mr Green "fleetingly" touched young activist Kate Maltby's knee during a meeting at a pub in 2015 and a year later sent her a "suggestive" text message.
Mr Green said any allegation that he made sexual advances to Ms Maltby was "untrue (and) deeply hurtful".
The department is separately probing whether international trade minister Mark Garnier breached the ministerial code after he reportedly admitted asking his secretary to buy sex toys and calling her "sugar tits".
The Prime Minister will hold crisis talks with other Westminster leaders on Monday to discuss plans for tackling sexual abuse and harassment.
She said MPs from all parties are "deeply concerned" about allegations that have emerged in recent days as she invited political counterparts to talks on setting up a new "transparent, independent" grievance procedure.
"We have a duty to ensure that everyone coming here to contribute to public life is treated with respect," she told MPs at Prime Minister's Questions.