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
Showers turning increasingly wintry after dusk. Snow may settle over higher routes by dawn. Risk of ice for the Monday rush hour
Winds remaining strong today and dragging in colder air. A mix of sunshine & showers, these turning increasingly wintry over hills
Very windy with gales for many. Rain moving east