Peter Hain has announced he will stand down as a Labour MP at the general election.
The former Cabinet minister said he had "decided to draw stumps" on his Commons career following a series of discussions with Ed Miliband.
Mr Hain, who was one of the Labour leader's earliest backers in the battle for the party's top job, indicated that he may take on a new role away from Westminster.
Mr Miliband praised the Neath MP for his integrity, wisdom and tireless work for the country and said he would be sorely missed.
During his lengthy career, he served in the cabinet under Tony Blair and Gordon Brown, with stints as Northern Ireland s ecretary, Welsh secretary, work and pensions secretary and leader of the Commons.
Mr Hain had been reselected to fight the 2015 election for Neath but said he would now find "new ways" to be involved in politics.
" Having been reselected last December as Labour candidate for Neath, I was planning to stand again next year," he said. "However, after considerable thought and in discussion with Ed Miliband, leader of the party and for many years my close colleague, I have decided to draw stumps on my House of Commons career.
"I have been in politics for some 50 years, 23 of those in the House; whilst it is my firm intention to remain active in politics, I have decided that it is time to find new ways to represent the new progressive politics to which I and Ed Miliband are committed. It's time for a change.
" By next May I will have been privileged to serve the marvellous constituency of Neath and its people for nearly a quarter of a century. They warmly welcomed me as an outsider, and members of my family subsequently moved to the Neath Valley where I will remain living after I have stepped down and continue to support Labour in our mission to build a better Neath.
"Meanwhile I will campaign hard once my successor has been chosen for a Labour victory - in Neath, throughout Wales and across Britain."
Mr Hain was raised in South Africa and first came to public attention as a hardline anti-apartheid campaigner before entering mainstream politics.
He angered Middle England with his role in disrupting South African rugby and cricket tours. In 1972 he was sent a letter bomb, but it failed to explode because of faulty wiring.
Former Labour leader Neil Kinnock persuaded him to join the Labour party and he became MP for the rock-solid Labour seat of Neath in a by-election in 1991.
When Labour stormed to power in 1997 he became a whip and then a junior minister in the Welsh Office. Tony Blair was impressed by his modernising skills and also the effort he put into the referendum campaigns for devolution in Scotland and Wales.
In October 2002 Mr Hain replaced Paul Murphy as Secretary of State for Wales and was then given the additional role of Leader of the Commons in June 2003.
He became Northern Ireland Secretary in May 2005, working with Mr Blair and then Irish taoiseach Bertie Ahern to persuade all parties to end the political deadlock in the province.
In 2007 he threw his hat in the ring for the deputy Labour leadership but failed to properly register donations to his campaign, leading him to quit as work and pensions and Welsh secretary the following year.
In 2009 he returned to government as secretary of state for Wales.
Mr Miliband said: "A political activist and campaigner for over 50 years, Peter Hain is one of the most experienced politicians in the House of Commons in which he has served as Member for Neath for nearly a quarter of a century.
"It goes without saying that his integrity, wisdom and firmness in speaking up for those least empowered to speak for themselves, will all be sorely missed. Whether on the backbenches, the front bench or in the Cabinet he has served the Labour Party and the country tirelessly.
"He served with great distinction as Secretary of State for Wales, for Work and Pensions and as Leader of the Commons but it was as Secretary of State for Northern Ireland that Peter will be most missed, negotiating the historic peace settlement establishing devolved government in North Ireland.
"Whilst I will miss his personal friendship and humour on the green benches, and he will be greatly missed by the people of Neath, I know he will continue to serve our party and the country and remain actively engaged in politics and public life for many years to come."
Neath's Constituency Labour Party paid tribute to Mr Hain's service.
In a statement it said: "For the last 23 years Peter Hain has served the people of Neath tirelessly, fighting for constituents and our Labour Party values. Holding government positions like secretary of state for Wales he delivered further devolution for the people of Wales and at the department of trade and Industry he brought many hundreds of millions of pounds of compensation to thousands of chronically ill miners.
"His unstinting work to negotiate a peace settlement in Northern Ireland has brought a stability to the area that just years before no-one would have thought possible.
"In the Neath constituency he has helped many thousands of people through his surgeries and advice office. For over two decades he has been a popular campaigning MP fighting for residents and taking up the causes with great passion and commitment, and putting Neath on the map.
"We thank Peter unreservedly for what he has done for us and are sure he will continue to play an active role in the constituency and the party.
"As a constituency party we now look to begin the process of finding our parliamentary candidate for the 2015 general election and winning a Labour majority."