A long-awaited report by the inquiry into the Iraq War may not be published this year, according to reports.

BBC Newsnight said the much-delayed findings of the Iraq Inquiry into the background to the 2003 conflict had been held up by the process of Maxwellisation - giving individuals criticised in the report a chance to respond - which had become a "nightmare".

Newsnight's defence and diplomatic editor Mark Urban quoted a source close to the investigation as saying "nobody thinks it will come out this year".

The inquiry's chairman Sir John Chilcot has been forced to defend delays after he said in January it would not be finished before May's General Election.

Prime Minister David Cameron has been among those expressing frustration that the report has yet to be finalised, more than five years after the inquiry, which took evidence from its last witness in 2011, was launched.

The BBC quoted the source as saying "once they had failed to meet the pre-election deadline, they gave up trying to speed things up".

Mr Urban said some of those criticised in the report had been given drafts running to "hundreds of pages", while others "engaged in lively correspondence with the inquiry team" in order to defend themselves.

He said: "Those I have spoken to speak of poorly-drafted letters from the inquiry that have required clarifications and given those disputing the inquiry's findings room to deploy their own arguments. Some have engaged lawyers to assist with their defence."

In February, Sir John told MPs: "My committee and I want, and intend, to deliver our report to the Prime Minister as soon as we possibly can. But as I said to the Prime Minister ... I see no realistic prospect of doing so before the General Election.

"It is our duty to deliver a report which gives the Government, Parliament, the public, and particularly all those who have been deeply affected by events in Iraq, the answers they deserve."

Deputy Prime Minister Nick Clegg said he was "dismayed beyond belief" at the reported delay.

Speaking to ITV News on the election trail in St Ives, he said: "I'm just dismayed beyond belief that we are having to wait so long - and now, it is being reported, even longer than I had feared - to finally find out the truth of what happened in the run-up to that fateful - and what I have always considered, personally, to be illegal - invasion of Iraq."