The six-year-longBritish inquiry into the 2003 Iraq invasion and its aftermath will not be published before the general election, prompting an outcry from those demanding that the long overdue reckoning should be put before the voters.

Sir John Chilcot, the chairman of the inquiry, will set out his reasons for the further postponement in an exchange of letters with David Cameron on Wednesday. The inquiry was set up in 2009 and took public evidence from its last witness in 2011.

The prime minister has already expressed his personal frustration at the repeated delays, and a cross-party group of backbenchers had been due to stage a debate and vote in parliament on 29 January, demanding publication before the election.

Tony Blair, the prime minister at the time of the war, has insisted he is not the culprit behind the delay in publication; his allies have suggested the blame lies with the civil service and sensitivities about the relations between the UK and US intelligence agencies.

There has been a stand-off between those demanding that the personal exchange of messages between the former US president George W Bush and Blair in the run-up to the war be published, and those saying such a move would represent an unprecedented breach of confidence concerning one of the most sensitive episodes in British foreign relations.

It is understood the publication date of the inquiry was discussed by the UK and American delegations when Cameron met Barack Obama at the White House last week. But the threat of a Commons vote will have added urgency to the issue.

In June last year Chilcot announced he was satisfied that the “gist” of talks between Blair and Bush could be made public, removing a big obstacle to publication of his report. Chilcot is understood to have sent “Salmon letters” to those who were to be criticised to give them an opportunity to respond before the report’s publication, which will have led to further delays following objections from those criticised.

The deputy prime minister, Nick Clegg, reacted furiously, saying the public, soldiers and families affected needed closure after six years of delay, adding that the public will think the findings are being “sexed down” to meet the needs of the establishment.

In a letter to Chilcot, he said: “I welcome your efforts to ensure the inquiry has been methodical, rigorous and fair in its approach. I also support your efforts to allow individuals criticised in the report to see the draft criticism and make representations to the inquiry before publication.

“However, neither administrative processes nor a constant back and forth between the inquiry and witnesses criticised should frustrate an independent report so important to the country’s future from being published as soon as possible.

“The public have waited long enough and will find it incomprehensible that the report is not being published more rapidly than the open-ended timetable you have now set out.

“We need to see a much clearer and more defined timetable, known publicly, with strict deadlines and a firm date for publication.

“If the findings are not published with a sense of immediacy, there is a real danger the public will assume the report is being ‘sexed down’ by individuals rebutting criticisms put to them by the inquiry, whether that is the case or not.”

Angus Robertson, the SNP’s Westminster leader said: “If Chilcot is to be delayed again it would be an absolute scandal.”

Blair previously said he wanted the Chilcot report to be published as soon as possible and that he resented claims he was to blame for its slow progress.

He has made repeated attempts to justify the highly controversial invasion, but has conceded that, for a variety of reasons, including disputes in the Bush administration, the detail and quality of post-war planning was inadequate.

Blair is determined to rebut the argument that he lied to parliament over the intelligence he had been given over the likelihood that Saddam Hussein possessed weapons of mass destruction. The basis of this claim and the key informants have emerged and been discredited. Ministers have conceded that if the final report were not completed by the end of February, it would be wrong to release it in the heat of a closely fought election campaign.

Although Ed Miliband was not in parliament at the time of the invasion, and has said he would have opposed the war, Labour probably has least to gain from the reopening of the debate about the basis of the invasion and its continuing consequences, including the rise of Islamic State, or Isis.

The Conservatives, including an agonised Cameron, backed the invasion at the time, but the Tories subsequently said they had been misled about the intelligence. Although Cameron pushed through military action in Libya, and, in principle, air strikes to punish Bashar al-Assad’s use of chemical weapons in Syria, the prime minister has generally been a sceptic about humanitarian military action. The Liberal Democrats opposed the war and probably would gain most politically from publication.

David Davis, the former shadow home secretary who has been a leading voice in calling for the report to be published before the election, said it was incomprehensible that the report was being delayed until after the election.

Davis told the Guardian: “Frankly this is not good enough. It is more than five years since it started. It is incomprehensible as to why this is [being delayed]. We need to know why. This is not simply some formality. This is for the whole country to understand why we made a terrible mistake in Iraq. Simply putting it off is not good enough.

“Why has this taken so long? What is going on that is preventing this? The report was created in the first place by a Labour government in order to get an understanding of what went wrong. I can think of no reason why this should be deferred.”

Davis has been a driving force behind the backbench Commons vote next week that would call on Chilcot to publish in a few weeks. He said the vote would not bind Chilcot in case there was complex legal justification for the delay. But Chilcot would have been expected to explain to MPs the delay. “We are getting neither. We are getting neither the report nor the explanation,” he said.