Legal letters to Labour figures, such as Tony Blair, are yet to be sent out

The £10 million Iraq War inquiry is set to be delayed until after the election because of continued Whitehall wrangling over what it will be allowed to publish.

Whitehall sources said inquiry chief Sir John Chilcot is still making a ‘steady stream’ of requests asking civil servants to declassify documents – suggesting that the report is still not written.

It also emerged that legal letters to key figures like Tony Blair and Alastair Campbell, spelling out criticism they may face in the report, have not yet been sent out.

The full transcripts of phone calls and letters between George W Bush and Tony Blair will not be published in Chilcot's final report, to the dismay of the families of the 179 British soldiers killed in the conflict

The report is expected to be critical of the actions of key Labour figures in the months before the Iraq War

The revelations are a major blow to hopes that the inquiry, which began work in 2009, would finally publish its report before the election next May.

Cabinet Secretary Sir Jeremy Heywood is said to have warned the inquiry that the report, which is expected to be critical of Mr Blair and some senior Labour figures, should be published by the end of January to avoid interfering with the election.

He is thought to have set an absolute deadline of early March, after which time the report would have to be delayed to prevent it becoming a political football during the election campaign.

The development will dismay families of the 179 British soldiers killed in the bloody conflict who have waited years to discover the real reasons their loved ones were sent to war.

Some have already branded the inquiry a ‘whitewash’, after it emerged that it will not be allowed to publish full details of 25 letters Mr Blair sent to George W Bush or the transcripts of 130 phone calls between the two men.

Sir John Chilcot's inquiry began work in 2009, but is still thought to be months away from being published

In them, Mr Blair is said to have promised the US President unconditional support for the war, telling him: ‘You know, George, whatever you decide to do, I’m with you.’

Sir Jeremy ruled in May this year that only the ‘gist’ of these conversations could be published.

The deal, which followed two-and-a-half years of wrangling, was seen as a breakthrough that might finally allow the report to be published.

But, to the dismay of some in Whitehall, Sir John has continued to make fresh requests to publicise sensitive information, each of which takes weeks to consider.

There is also mounting speculation that Sir John is struggling to balance the views of the inquiry panel on some key issues, suggesting that his report may not be completed for months.

A spokesman for the inquiry confirmed that the legal process in which figures like Mr Blair will be given the chance to respond to criticism has not yet started. This process is expected to take at least two months and must be completed before the report can be released.

Sir Jeremy acknowledged last week that there had been a ‘delay of sorts’ while thousands of requests to declassify documents were considered.

But he insisted there would not be a cover-up.

Giving evidence to MPs he said: ‘I am absolutely confident that the report will be as transparent as it needs to be.

‘I can understand people’s frustration but I think the report will be more transparent than you are expecting, and I don’t think in any shape or form it can be called a cover-up