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
A pensioner has been told he is facing jail for attacking three off duty nurses who were trying to help him.
Manchester Arena has seen one of the busiest periods in its history since reopening, venue bosses say.
Residents in Lancashire are being encouraged to prepare for floods, and to check if they need to take steps to protect their homes.