Former Prime Minister David Cameron is standing down as an MP, triggering a by-election in his Witney seat.
Mr Cameron, who quit as Prime Minister shortly after the EU referendum, said he had "thought about (his future as an MP) long and hard over the summer", deciding it was now "the right thing to do to stand down" from his constituency in Oxfordshire.
He added: "In my view, in modern politics, with the circumstances of my resignation, it isn't really possible to be a proper backbench MP as a former Prime Minister.
"I think everything you do will become a big distraction and a big diversion from what the Government needs to do for our country."
Mr Cameron did not give any indication as to what he might do with his life after politics, saying he has yet to make any "firm decisions".
However, he said he would continue to live in the constituency and support local causes "but obviously I'm going to have to start to build a life outside Westminster".
"I hope I'll continue to contribute in terms of public service and of course contribute to this country I love so much," he added.
He said he had spoken to current Prime Minister Theresa May about his decision and she had been "very understanding".
When asked if Mrs May's plan to introduce a new series of grammar schools was a factor in his decision, he replied: "This decision has got nothing to do with any one individual issue.
"The timing in that way, I promise, is coincidental."
Mrs May said in a statement: "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.
"I thank him for everything he has done for the Conservative Party and the country and I wish him and his family well for the future."
Mr Cameron, 49, was first elected as Witney's MP in 2001, becoming Conservative leader after the 2005 General Election and entering No 10 as coalition Prime Minister in 2010.
In March, he had said he intended to stand again for Witney, telling the BBC: "I love being MP for Witney and am very keen to continue".
He resigned as Prime Minister on 24 June, just hours after Britain voted to leave the European Union.
Unison general secretary Dave Prentis said: "David Cameron led us into a referendum he didn't want and then lost.
"He walked away from the country in June and now he's done the same to his constituents."
Liberal Democrats leader Tim Farron said Mr Cameron "deserves credit" for forming a coalition government with his party between 2010 and 2015.
But, regarding the EU referendum, Mr Farron said: "He took a politically motivated and short-termist gamble with our country's future and lost, to the huge detriment of future generations.
"He now leaves the rest of us to pick up the pieces."