David Cameron has warned he is "fast losing patience" with the Iraq inquiry after chairman Sir John Chilcot insisted he still cannot say when it will report.
In an exchange of letters, Sir John told the Prime Minister that responses submitted by individuals mentioned in the report had "opened up new issues". Others have still yet to reply.
"It is now essential that all remaining responses are received so that the process can be completed," he wrote. "Only when all responses are in our possession and have been evaluated will I be able to write to you with a realistic timetable for completion."
But Mr Cameron expressed "disappointment" and said the public and those who lost loved ones were "awaiting the results of your work".
"They, and I, had hoped for publication of your report by now and we are fast losing patience," he added.
Suggestions that the inquiry, which was launched in July 2009, may not report for another year have prompted calls for Mr Cameron to intervene to bring it to a close.
Sir John made clear that the process of Maxwellisation, where draft criticisms are put to individuals and they are given the chance to respond, is not complete.
His reference to replies opening up "new issues" and referring to "material that was not part of the evidence submitted to the inquiry" will also fuel speculation about a substantial further delay.
Former prime minister Tony Blair has dismissed reports that he is among those who have been delaying completion.
Acknowledging Mr Cameron's frustration with the slow progress, Sir John wrote: "I and my colleagues understand your concern that it has not been possible to publish our conclusions before now.
"I am sure you will also share my desire to ensure that those conclusions – covering a period of nine years – hold firm once published."
Standing in for Mr Cameorn at PMQs in the Commons, Chancellor George Osborne called on those involved in the inquiry to "get on with it" and publish the report.
He spoke as SNP Westminster leader Angus Robertson said ministers have a "moral or political responsibility" to find out why the report keeps being delayed.
Mr Osborne said: "The Chilcot Inquiry is of course completely independent of Government and we do not determine when it publishes its conclusions.
"But where I agree with you is this - it has been a long time coming and people I think are running out of patience, they want to see that report.
"I'd make a broader observation that of course there was a cross-party alliance between the Scottish Nationalists and the Conservative Party when we called for that inquiry to be set up earlier than it actually was.
"If it had been we'd have the conclusions now."
Mr Robertson went on: "It's worth remembering that the Chancellor of the Exchequer and the Prime Minister both voted for the war that we were led into by the then Labour government.
"Do you feel that you have no moral or political responsibility to get to the bottom of the reasons why we went into this catastrophic war in the first place?
"And what are you going to do about it?"
Mr Osborne replied: "That responsibility was fulfilled when we voted to create that independent inquiry, we want to see the results of that independent inquiry, I think those involved in Chilcot will have heard the view of the House of Commons today and indeed public concern about how long this inquiry is taking.
"But ultimately it is an independent inquiry, if it wasn't an independent inquiry, people would question its motives and the basis upon which it had been set up. It is independent but I think it should get on with it."