Labour MP Shaun Woodward, who was once sacked by William Hague from the Tory party front bench for backing a repeal of Section 28, is to step down from Parliament at the 2015 general election.
The former Northern Ireland secretary said he wanted to pursue voluntary and business interests after 18 years spent in the Commons.
Shaun Woodward has long supported the rights of gay and lesbian people in the UK.
In 1999, he was also sacked from the front bench of the Tory party for backing a repeal of section 28, which he said might have lead to teachers being unable to deal with homophobic bullying within schools.
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 added his defection to the Labour party had taken “personal courage and political commitment” and that Labour “will be forever grateful to you”.
Writing to Mr Miliband to confirm his decision to quit, he said he had been a “One Nation” Conservative and that the idea, which is now adopted by the Labour leader, was “at the core of my political beliefs”.
Mr Miliband responded: “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.”
Following his departure, Mr Woodward said he would focus on human rights issues, including his role as part of a Privy Council review of the use of intercept evidence in court.