St Helens South and Whiston MP Shaun Woodward, who became a Labour cabinet minister after defecting from the Conservative Party, is to step down at the 2015 general election.
The former Northern Ireland secretary said he wanted to pursue voluntary and business interests.
Ed Miliband said he had served with "great distinction" as an MP and minister and paid tribute to the "most acute strategic advice" he provided to Labour leaders after crossing the floor.
He said he would focus on human rights issues after leaving Parliament - including as part of a Privy Council review of the use of intercept evidence in court.
In 2010 he won his seat with a majority of 14,122
More top news
Sir Ian McKellen is one of the most recognisable actors in the world. He is returning to his hometown of Wigan to receive his very own star.
The Care Quality Commission says other hospitals can learn from the example of Salford Royal so what's it getting right?
Cloudier skies from the west, but mainly dry. More unsettled for the weekend