Douglas Feith, a top policy adviser to US Defence Secretary Donald Rumsfeld and a driving force behind the Pentagon's Iraq policy, is to resign.
Feith was a driving force behind America's policy in Iraq
The Pentagon says Mr Feith - its number three civilian official - is stepping down for personal and family reasons.
He has been one of the most controversial members of President George W Bush's administration.
Critics say the office he oversaw misrepresented intelligence on Iraq before the war.
They claim the Office of Special Plans fed policy-makers uncorroborated pre-war intelligence on Saddam Hussein's Iraq, particularly on suspected ties with the al-Qaeda network - a charge Mr Feith and the Pentagon have always denied.
Identified as a leading hawk, Mr Feith was one of the driving forces behind the Pentagon's planning for the Iraq conflict, says the BBC's Pentagon correspondent Nick Childs.
He has also been instrumental in a number of key Pentagon reforms including a major overhaul of US forces around the world, that will see significant withdrawals from Germany and South Korea.
In a statement, Mr Rumsfeld expressed regret over Mr Feith's decision and said he would be missed.
"I'm hopeful he'll stay until we find an appropriate successor. We have not started looking for one," Mr Rumsfeld said.
It is not exactly clear when Mr Feith will leave his job, but it is expected to be in the next few months, our correspondent says.