In January, inquiry Chairman John Chilcot said he was frustrated by a decision from Cabinet Secretary Gus O'Donnell to keep extracts of notes between British Prime Minister Tony Blair and U.S. President George W. Bush secret.
Chilcot in his statement said the inquiry recognized the sensitivity of bilateral exchanges between heads of state but was asking for "key extracts" to highlight Blair's position at "critical points."
The inquiry, in a statement Thursday, said it completed its public hearings and was examining written and oral evidence in preparation for a final report.
"The inquiry has advised the government that it will need until at least summer 2012 to produce a draft report which will do justice to the issues involved," it said in a statement. A final report was expected by the end of the year.
Chilcot's panel said it needed to negotiate the declassification of a "significant volume" of secret documents with Prime Minister David Cameron's government to include them in the final report.
British Prime Minister Gordon Brown in 2009 called for an inquiry to examine the eight-year period beginning with the push for war in 2001 to July 2009 when British forces ended their mission there.