MP Shaun Woodward, who became a Labour cabinet minister after defecting from the Conservative Party, is to step down from Parliament at the 2015 general election.
The former Northern Ireland secretary, who represents St Helens South and Whiston, said he wanted to pursue voluntary and business interests after 18 years in the Commons.
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.
The ex-BBC producer was press chief at Conservative Central Office before becoming MP for Witney - the safe Tory seat now held by David Cameron - in 1997.
He was made a front-bench spokesman under William Hague b ut was sacked in 1999 for supporting the repeal of Section 28, a regulation which prevented the promotion of homosexual lifestyles in school.
Soon after that he crossed the floor, refusing to quit as MP for Witney, and in the 2001 general election was handed a safe Labour seat which he won despite a strong left-wing challenge in protest.
After his re-election in 2005 he was appointed a junior Northern Ireland minister before moving to culture, media and sport with responsibilities for the d igital TV switchover.
In 2007 he was a surprise pick as Northern Ireland Secretary in Gordon Brown's first cabinet - a role for which the wealthy politician, married to Camilla Sainsbury of the supermarket family, took no salary.
Writing to Mr Miliband to confirm his decision to quit, he said he had been a "One Nation" Conservative and that the idea - now adopted by the Labour leader - was "at the core of my political beliefs".
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.
Mr Miliband said his defection had taken "personal courage and political commitment" and that Labour "will be forever grateful to you".
He said he had made a "vital contribution" to the Northern Ireland peace process.
"You have been a great believer in One Nation throughout your career and I am delighted we can count on you as we fight to ensure that it is a One Nation Labour government which serves the country from 2015."
Mr Woodward failed to secure election to the shadow cabinet after Mr Miliband became party leader in 2010 but was kept as shadow Northern Ireland secretary as a discretionary appointment of the leader.
He lost the role in a 2011 reshuffle.
In 2010 he won his seat with a majority of 14,122.