The head of the public inquiry into the 2003 invasion of Iraq has repeatedly refused offers of help to speed up the completion of his long-awaited report.

Sir John Chilcot, who heads the Iraq Inquiry, told civil servants that they did not need extra resources despite mounting frustration over the delays in finalising his investigation.

Cabinet Secretary Sir Jeremy Heywood said there was nothing he could do to accelerate the process, but did not think the delays were deliberate.

The inquiry into the Iraq War started in July 2009 but the final report has been held up due to the process of Maxwellisation; where draft criticisms are put to individuals and they are given the right to reply.

Tony Blair, the former Prime Minister, has repeatedly insisted he is not to blame for the delay in publication. More than two dozen of his notes to former US President George W Bush, “subject to a small number of essential redactions”, are expected to be published alongside the report.

Today, the ex-Labour leader will be quizzed at an event being held by the Progress campaign group in London. It is likely he will be asked about the continuing delay in the publication of the Chilcot report.

Prime Minister David Cameron has expressed frustration with the inquiry not due to report until the turn of the year.

Under questioning at the Commons Public Administration and Constitutional Affairs Committee, Sir Jeremy said: "I have repeatedly offered to Sir John extra resources on behalf of the Prime Minister, extra legal resources and so on...They say they don't need them; they are doing it as fast as they can."

A spokeswoman for the inquiry said: "Sir John and Sir Jeremy discussed the additional assistance the inquiry will wish to call upon in its closing stages and agreed some steps that can be taken now to assist the inquiry in its work."