The Iraq Inquiry report will be delayed by at least six months because of a row over the release of secret documents, it has emerged.
A statement on the inquiry's website blamed the delay on getting hold of papers revealing decisions taken in the run-up to war in 2003.
The statement says: "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.
"Very considerable progress has already been made, but there is still much to be done.
"As well as drafting the report, the Inquiry will need to negotiate the declassification of a significant volume of currently classified material with the Government, to enable this to be quoted in, or published alongside, the Inquiry's report.
"That process has begun, but there will be a series of further major requests as drafting progresses. The Inquiry has made clear that it will need co-operation from the Government in completing this in a satisfactory and timely manner."
The inquiry, chaired by Sir John Chilcot, had hoped to publish its final report at the end of this year.
A Cabinet Office spokeswoman said: "These are wide ranging and complex issues, for the Government as well as for the inquiry.
"We are cooperating fully with the inquiry in completing its task."