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
Cloudy, windy and mild. Rain developing, especially across the hills - where fog and poor visibility will continue
Former Blackpool, Tranmere, Blackburn, Stockport and Oldham star quits football for a career in teaching
We are 'flipflopping' from mild weather to colder weather during the week leading up to Christmas, so what about big day itself?