Former Labour cabinet member Jack Straw is to step down from his current role, ending 30 years of frontbench politics.
The Blackburn MP has held many of the top jobs in British politics, including foreign secretary and home secretary.
He said he would quit as shadow justice secretary in early October when a new shadow cabinet had been appointed under a new Labour leader.
He said he planned to write a memoir and spend more time in his constituency.
He has said he believes Labour needs a "fresh start".
Announcing his resignation from front-line politics, the veteran MP said: "I was first appointed to the Labour front bench in 1980, and then elected to the shadow cabinet in 1987.
"But now I want the freedom to range more widely over foreign and economic policy."
He told BBC Radio Lancashire that after his recent 64th birthday "I just thought it was time for a change".
"There is something relentless about the front bench, whether it's in government, or in opposition, and I'd like to do other things," he added.
He said said he hoped his memoir would be "readable" and not "tedious or self-serving" - but he would not be following in the footsteps of his former cabinet colleague Lord Mandelson, whose recent book dominated the headlines with tales of infighting at the top of New Labour.
"I don't approve of people breaking confidences. It may sound very old-fashioned, but I don't approve, for example, of the way Peter Mandelson has behaved and neither do quite a number of my colleagues."
He said he would be announcing which of the five candidates he would be backing in the Labour leadership contest next week.
Mr Straw is a former president of the National Union of Students, who stood unsuccessfully in the February 1974 general election before going on to enter Parliament in 1979.
He practised as a criminal barrister before entering the Commons and was an adviser to Labour minister Barbara Castle, his predecessor as Blackburn MP.
Known as one of the great political survivors, he is one of a handful of Labour MPs to have served in cabinet throughout the party's 13 years in power.
He was Tony Blair's first home secretary, in 1997, and replaced Robin Cook as foreign secretary after the 2001 general election.
As foreign secretary, he played a central role in the decision to commit British troops to the US-led invasion of Iraq and in unsuccessful attempts to secure a second UN resolution on the eve of war.
In evidence to the Chilcot inquiry in January, he described his decision to back the 2003 war as the "most difficult" of his career, describing it as a "profoundly difficult political and moral dilemma".
He was appointed Leader of the House of Commons in Mr Blair's May 2006 reshuffle, steering through modernising reforms.
A year later he took charge of Gordon Brown's campaign for the Labour leadership, after performing a similar role for Tony Blair in 1994.
He was rewarded with the role of Lord Chancellor and Secretary of State for Justice, after Mr Brown gained the leadership unopposed, making Mr Straw the first Lord Chancellor to be a member of the Commons since the 16th Century.