The Iraq Inquiry is making "significant progress", although it still cannot set a date for the long-awaited publication of its findings, the inquiry chairman has said.
Sir John Chilcot said officials were continuing to work through the so-called "Maxwellisation" process of assessing responses from individuals facing possible criticism in the final report.
He played down a claim by Cabinet Secretary Sir Jeremy Heywood that he had turned down repeated offers of assistance to speed up the publication.
In a letter to the Commons Foreign Affairs Committee, Sir John said inquiry officials had had a "constructive discussion" about the additional assistance the inquiry would need in its final stages.
The continuing delays to the inquiry - which was launched in 2009 - has caused exasperation among MPs with David Cameron warning last month that he was "fast losing patience" over the lack of progress.
In his letter Sir John said that while the Maxwellisation process had made "significant progress", the responses from the individuals involved had to be properly evaluated before the work could be concluded.
"At that point I shall write to the Prime Minister with a timetable for completion and delivery of our report," he said.
He added: "Those involved in the Maxwellisation process have engaged fully, and I continue to judge that no one has taken an unreasonable length of time to respond given the range and complexity of the issues under consideration."
Sir John confirmed that he met Sir Jeremy earlier this month following an exchange of letters with the Prime Minister.
"Sir Jeremy and I had a constructive discussion about some steps that can be taken now to assist us in our work and the additional assistance the inquiry will wish to call upon in its closing stages," he said.
Earlier this week Sir Jeremy told the Commons Public Administration and Constitutional Affairs Committee that the inquiry had repeatedly been offered additional resources but they had always been turned down.