Jack Straw is to step down from frontbench politics after 30 years, he announced today.
The Blackburn MP, who served as Justice Secretary, Home Secretary, Foreign Secretary and Leader of the Commons, said he believed Labour needed a "fresh start".
"I was first appointed to the Labour frontbench in 1980, and then elected to the Shadow Cabinet in 1987," he told the Press Association. "But now I want the freedom to range more widely over foreign and economic policy."
He added: "I'll be spending more time on my constituency, and I will be writing a memoir in time."
Mr Straw said he would leave in early October when a fresh shadow Cabinet had been appointed under a new Labour leader.
Mr Straw said he was not planning to quit as an MP, pointing out that he had been returned with a higher majority at the General Election. "They seem to want me," he added.
Mr Straw, Gordon Brown and Alistair Darling were the only Labour figures to remain in the Cabinet throughout the party's 13 years in power. Mr Darling has already indicated he is retiring to the backbenches.
Mr Straw told the Lancashire Telegraph he had been "extraordinarily lucky" with his career.
"My decision (to leave the frontbench) was entirely voluntary. I decided a few months ago, as it seemed we were going into Opposition, that I wouldn't stand.
"The Shadow Cabinet is about offering a fresh start for a party in opposition to present itself to the electorate and I thought 30 years was getting on for enough."
He insisted his memoirs would not be "kiss and tell", a charge that has been levelled at the recent book by former Business Secretary Lord Mandelson.
"I'm going to take a bit of time, and do them differently," Mr Straw said. "I'm afraid I don't agree to people breaking confidences."
Mr Straw entered Parliament in 1977, and was made Opposition Treasury spokesman in 1980.
Neil Kinnock promoted him to the Shadow Cabinet as education spokesman in 1987, and when Tony Blair became leader in 1994 he was appointed shadow home secretary.
He took charge of the Home Office after Labour gained power in 1997, controversially bolstering anti-terror powers and introducing Asbos - but also steering through the Human Rights Act.
In 2001 Mr Straw became Foreign Secretary and oversaw the fraught negotiations in the run-up to the Iraq War.
However, he was demoted to Leader of the Commons in 2006 amid friction with the PM over policy towards Iran.
When Mr Brown entered Number 10 the following year, Mr Straw was made Justice Secretary and Lord Chancellor.
Despite periodic speculation that he could be installed as a "caretaker" PM if Mr Brown was ousted, he remained in the post until Labour was ejected from government.