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
NHS staff to be offered Zumba classes and counselling to keep them fit and in work, NHS England's chief executive is to announce.
Manchester United goalie De Gea 'must play' to get picked for Spain - says the Spanish manager Vincente Del Bosque.
100,000 renters in the North West could be victim to 'rogue' landlords according to new research.