British Prime Minister Gordon Brown in 2009 called for an inquiry to examine the 8-year period beginning with the push for war in 2001 to July 2009 when British forces ended their mission there.
A final report from the inquiry, led by John Chilcot, was expected this year. Chilcot, however, said the report wasn't expected until the middle of next year.
"Writing a report covering so wide and complex a time period necessarily takes time," he said in a statement. "Whilst writing the report, we are also simultaneously seeking the declassification of much relevant material so the public will understand why and how the Inquiry has reached its conclusions."
The inquiry last week published 40 letters, testimonies and transcripts related to the investigation. In a letter to British Prime Minister David Cameron, Chilcot said there were "a number of particularly important categories of evidence" that needed to be addressed.
Chilcot last year 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.