Sir John Chilcot told the Prime Minister last month that he still could not say when the inquiry would report, prompting David Cameron to say he was “fast losing patience” over the lack of progress.
Cabinet Secretary Sir Jeremy Heywood said there was nothing he could do to accelerate the process, but insisted that he had offered Sir John extra resources.
Sir Jeremy said: “I’m not washing my hands of it. It is an independent inquiry, the timetable is not in my hands.
“I have repeatedly offered to Sir John extra resources on behalf of the Prime Minister, extra legal resources and so on.
“At the Prime Minister’s request I saw him again recently, we had a private meeting at which I repeated that request, obviously.
“I just know that John Chilcot will complete this report as soon as he possibly can. He is as aware as everybody else is about the importance of getting this done and quickly.”
He added: “We have repeatedly offered the inquiry further resources, they say they don’t need them, they are doing it as fast as they can.”
The inquiry into the Iraq War started in July 2009 but the final report has been held up for the process of Maxwellisation, where draft criticisms are put to individuals and they are given the chance to respond.
Under questioning from Labour MP Paul Flynn at the Public Administration and Constitutional Affairs Select Committee, Sir Jeremy said: “Everybody shares your frustration, from the Prime Minister downwards - including Sir John Chilcot - about how long this has taken.”
Sir Jeremy insisted: “I don’t think anybody is deliberately trying to slow down the inquiry,” but added that there would be a “long, hard look” once it had completed its work to find out why it took so long.
“But not in a way that interrupts the last phase of the inquiry, the inquiry needs to just get its head down and complete its work,” he said.