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.
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.
During his lengthy career, he served in the cabinet under Tony Blair and Gordon Brown, with stints as Northern Ireland secretary, Welsh secretary, work and pensions secretary and leader of the Commons.
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.
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 Labour seat of Neath in a by-election in 1991.
In October 2002 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.
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."