Stephen Crabb has resigned as work and pensions secretary after just four months in the role.
He has been replaced by Ashford MP Damian Green.
A statement from Crabb says: "After careful reflection I have informed the Prime Minister today that, in the best interests of my family, I cannot be part of her government at this time.”
Crabb says he looks forward to “supporting the government's One Nation vision from the backbenches”.
He was appointed work and pensions secretary in March, following the resignation of Iain Duncan Smith.
Over the weekend, The Times reported Crabb had been sending “flirtatious and sexually charged” WhatsApp messages to a woman he met through his political role several years ago.