LONDON (AFP) - A long-awaited public inquiry into Britain's role in the Iraq war will not be published until after an election in May, it was reported Wednesday, sparking criticism of the delay.

Dubbed the Chilcot Inquiry after its chairman John Chilcot, the probe was announced in 2009 to investigate the justification and conduct of the six-year conflict, and was intially expected to be released in 2010.

Publication has been slowed by disagreement over the release of communications between former British prime minister Tony Blair and former US president George Bush, as well as by a process to allow those criticised in the report to respond.

"The public have waited long enough and will find it incomprehensible that the report is not being published more rapidly," Deputy Prime Minister Nick Clegg wrote in a letter to the inquiry chairman.

His comments were in response to news of further delay, which was widely reported in the British press.

Mr Clegg said an "open ended timetable" that meant the findings would not be published for months risked fuelling public perceptions "the report is being 'sexed down' by individuals rebutting criticisms".

"The Inquiry into Iraq will both resolve the issues of the past, and set the tone for future British foreign policy. We cannot wait any longer for these lessons to be learned," Clegg wrote.

Prime Minister David Cameron earlier this month said he felt "immense frustration" at continuing delays in the report.

The inquiry was charged with examining the justification for the invasion of Iraq in 2003 and its legality, the conduct of the war and supply of equipment to Britain's troops, and Iraq's descent into chaos after the invasion.

It is likely to be politically sensitive and is expected to be critical of Blair, who was called as a witness in the investigation and who has been forced to deny he is to blame for delays in its publication.

The inquiry had cost £9 million (S$18.1 million) by April 2014.