The former Prime Minister made the shock announcement today just weeks after promising to stay in Parliament until 2020.
The Witney MP quit 10 Downing Street - which he also swore he wouldn't do - on June 24 after a humiliating finish on the losing side of the EU referendum.
At the time he vowed: "I will continue with my duties as the MP for Witney. It is an enormous privilege to serve the people of West Oxfordshire."
But today he warned it was "very difficult to sit as a backbencher and not be an enormous diversion and distraction from what the government's doing." The full text of his statement below...
"I spoke this morning to the chairman of the West Oxfordshire Conservative Association and I have just come from a meeting with my constituency agent and other members of staff.
"Having fully considered my position over the summer, I have decided that I am going to stand down as the Member of Parliament for Witney.
"There will now be a by-election and I will do everything that I can to help the Conservative candidate win that election.
"In my view, the circumstances of my resignation as prime minister and the realities of modern politics make it very difficult to continue on the backbenches without the risk of becoming a diversion to the important decisions that lie ahead for my successor in Downing Street and the Government.
"I fully support Theresa May and have every confidence that Britain will thrive under her strong leadership.
"It has been an honour to serve the people of Witney and West Oxfordshire for the last 15 years. I want to thank them for making the job of representing them such a pleasure and a privilege.
"I would like to thank the party workers, volunteers, councillors and supporters for all they have done to help me.
"Our house in Dean is the place Samantha, my children and I call home, and that will not change. I will continue to support the local causes and charities that mean so much to me and many others in this beautiful and enterprising part of our country.
"I now look forward to a life outside of Westminster, but hope to continue to play a part in public service and to make a real and useful contribution to the country I love."