Armed forces minister Adam Ingram has quit the government after six years at the Ministry of Defence.
Mr Ingram was at the Ministry of Defence for six years
He was asked by Prime Minister Gordon Brown to head a year-long review into the military's role in tackling the global terrorist threat.
He has been a minister for 10 years, first for Northern Ireland and then at the MoD. His successor has yet to be named in Mr Brown's reshuffle.
Defence Secretary Des Browne was the only Cabinet member to retain his job.
Mr Ingram wrote to the PM of his new role: "This is an area of particular interest to me and I'm pleased to accept your offer."
The Ministry of Defence website reported that Mr Ingram told assembled civilian and service staff before he left the premises for the final time that it had been "a profound and unique experience to have worked in the MoD".
He said his many trips to UK troops stationed around the world had taught him "that this nation should be proud of those that serve in our name".
As well as praising the sacrifices that personnel made, he also said the civil servants also set "the highest standards of public service and duty".