The Iraq Inquiry's report will not be published until the second half of next year, it was announced today.
The head of the investigation, Sir John Chilcot, has written to David Cameron alerting him to the delay.
The findings about the run-up to the 2003 invasion and its aftermath had originally been expected by the end of last year. The timing was then put back to this summer.
But a statement published on the inquiry website this afternoon said: "Pulling together and analysing the evidence and identifying the lessons, for a report that covers so wide and complex a range of issues and a time period of some nine years, is a significant task.
"Very considerable progress has already been made, but there is still much to be done.
"As well as drafting the report, the Inquiry is negotiating with the Government the declassification of a significant volume of currently classified material, in order that it may be quoted in, or published alongside, the Inquiry's report.
"Work on this substantial task, which involves the detailed scrutiny of many thousands of documents, is already under way.
"Significant progress has been made, but there will continue to be a series of further requests as drafting progresses."
The statement said Sir John intended to start writing to people who were set to be criticised by the middle of 2013.
"The Inquiry's report will be submitted to the Prime Minister as soon as possible after that process is complete," the statement added.
"The Inquiry understands that it will then be published in Parliament."
The five-strong panel began work in 2009, and held its last public hearing in February last year.
Last November, it said it needed extra time to "do justice to the issue involved" and suggested conclusions could be delivered to the Prime Minister this summer.
In his letter to Mr Cameron, Sir John said: "The Inquiry has made extensive progress in drafting its report, which will comprise both a reliable account of the events and the inquiry's conclusions, but its task is not yet complete.
"The issues are complex and difficult and there are significant lessons. Some of those are specific to the circumstances of Iraq, but most have a more general application for the conduct of government.
"The final report is likely to be more than a million words."