Former British prime minister David Cameron is standing down as an MP, triggering a by-election in his constituency.
Mr Cameron said he had "thought about this long and hard over the summer".
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.
"Everything you do will become a big distraction, a big diversion from what the Government needs to do for our country."
Mr Cameron announced his resignation as prime minister in June, only hours after it became clear that Britain had voted to leave the EU.
His resignation triggered a leadership campaign in the Conservative party, with Theresa May ultimately becoming party leader and prime minister.
Mr Cameron said he had spoken to Mrs 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."
Referring to the EU referendum, Liberal Democrats leader Tim Farron said: "[Mr Cameron] 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."
However, Mr Farron added that the former prime minister "deserves credit" for forming a coalition government with his party between 2010 and 2015.
Mr Cameron is an MP for Witney in Oxfordshire, where a by-election will now have to take place.
