Peter Hain has told ITV News he will "play a different role" in politics after he steps down as MP for Neath next year, but refused to say what that role would be.
The 64-year-old former Welsh Secretary said he had made his decision to leave the House of Commons after discussion with Labour leader Ed Miliband.
Mr Hain said he will "continue to be active in politics", supporting Labour and Neath.
Asked whether he would become a member of the House of Lords, he said: "I'm not talking about any particular role I might perform."
Mr Hain told our reporter Tom Sheldrick that his early work in fighting apartheid, delivering extra powers for the Welsh Assembly and helping secure a political settlement as Northern Ireland Secretary were "the things I feel most proud about".
Neath's Constituency Labour Party has 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.
Peter Hain has announced he will stand down as the Labour MP for Neath at next year's 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 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 Welsh secretary, Northern Ireland secretary, work and pensions secretary and leader of the Commons.
Mr Hain had been reselected to fight for the 2015 election for Neath, but said he would now find 'new ways' to be involved in politics.