Britain's Iraq Inquiry said Wednesday it must delay its final report by at least six months because of negotiations regarding classified documents.
The inquiry, headed by Sir John Chilcot, is examining mistakes made before and after the 2003 U.S.-led invasion of Iraq.
It was expected to publish before the end of this year its report on whether then-Prime Minister Tony Blair's government overstated the case for invasion and failed to prepare for the task of nation building.
But the Iraq Inquiry said on its website Wednesday that it will need at least until summer 2012 to produce a draft report. It blamed the delay on getting hold of classified papers, saying the inquiry still needs to negotiate the declassification of a "significant volume" of material.
"Very considerable progress has already been made, but there is still much to be done," a message posted on the inquiry's website said. It has made clear that it will need government cooperation to get that done in a "timely manner."
The Iraq Inquiry won't assign blame or criminal liability but is expected to make recommendations for handling future conflicts.
Over two years, the inquiry took evidence from political leaders such as Tony Blair, military chiefs and advisers. It also met with bereaved families, visited the U.S. and France, and held private sessions with intelligence officers.
The final report will be submitted to Prime Minister David Cameron.