David Cameron has announced that he is resigning as an MP, after stepping down as prime minister in June following the EU referendum.
The move will trigger a by-election in his seat of Witney, Oxfordshire, which he has held since 2001.
In a statement on Monday, Cameron said: "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 back benches 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."
Right decision by David Cameron to leave Commons - former Prime Ministers are either accused of doing too little or being a distraction.
It's a big change of heart from 27 June when he promised to stay on as an MP until the next general election. In a statement then, he said: "I will continue with my duties as the MP for Witney. It is an enormous privilege to serve the people of West Oxfordshire."
Cameron was asked on ITV News whether he was snubbing the new prime minister with his decision to quit the Commons.
He replied: "I spoke to Theresa May and she was very understanding about this decision. I support her and I support what she’s doing. I think she’s got off to a cracking start.
"Obviously I have my own views about certain issues. People know that. That’s really the point. As a former PM it’s very difficult to sit as a backbencher and not be an enormous diversion and distraction from what the government is doing.
"I don’t want to be that distraction. I want Witney to have an MP that can play a full role in parliamentary and political life in a way I would find very difficult if not impossible.”
Sorry to see my great friend @David_Cameron stepping down - he loved being Witney's MP; I know how difficult this decision has been for him
May said in a statement on Facebook: "I was proud to serve in David Cameron’s government – and under his leadership we achieved great things. Not just stabilising the economy, but also making great strides in delivering serious social reform.
"His commitment to lead a one nation government is one that I will continue – and I thank him for everything he has done for the Conservative party and the country. I wish him and his family well for the future."