Contents of key conversations between Tony Blair and a bellicose George W Bush, who declares he is ready to "kick ass", are thought to be among documents relating to the Iraq war that the government is withholding from publication.

It emerged this week that the Cabinet Office is resisting requests from the Iraq inquiry, the body set up to draw lessons from the conflict, for "more than 130 records of conversations" between Blair, his successor, Gordon Brown, and Bush to be made public. In a letter to David Cameron, published on the inquiry's website, the committee's chairman, Sir John Chilcot, disclosed that "25 notes from Mr Blair to President Bush" and "some 200 cabinet-level discussions" were also being withheld.

The standoff between the inquiry and Sir Jeremy Heywood, the cabinet secretary, has been going on for five months and has meant that the "Maxwellisation process", in which politicians and officials are warned that they will be criticised in the report, is on hold.

As a result, a date for the final publication of the report has yet to be agreed, more than four years after the inquiry started.

Critics have claimed that the government is seeking to suppress embarrassing material that could harm the UK's relationship with the US. Elfyn Llwyd, Plaid Cymru's leader in Westminster, has said it is "absolutely unacceptable" for the records not to be published. Chilcot has described the delay as "regrettable".

It is understood that some of their contents have already been disseminated in the public domain via a high-profile play. A note detailing the contents of a confidential phone call between Blair and Bush is quoted in the play Loyalty, written by journalist Sarah Helm, the wife of Blair's former chief of staff Jonathan Powell. Helm has always stressed that her play, which is partly biographical and was performed at the Hampstead Theatre, where it was seen by many senior Whitehall figures, is fiction, but well-informed sources say some scenes are uncannily close to the truth.

In the play the Powell character takes a note of a conversation in which Blair originally promises Bush: "We will be with you whatever." However, the phrase is crossed out by the Powell character and replaced with the more vague pledge "you can count on us whatever", giving Blair a less definitive commitment in the note recording the conversation.

Blair's original promise, revealed in the play, will bolster his critics' claims that he had made up his mind to support the US months before the UN voted on whether to back the invasion.

The play also discloses a crucial conversation between Blair and Bush who were pushing for a second UN resolution to be approved on a particular Friday.

"Condi [Rice] and Colin [Powell] want to do it Friday," Bush explains. "You know why – because we've got to move to closure on this. I can call in the chips with Chile. There's a moment to do this and we're peaking nicely. But we need to close it down soon."

Blair responds: "Well, let me explain how I see it. We think Monday might be better. I want to take the Europeans with me if I can. So Friday might be a little early for us."

Earlier in the conversation Bush tells Blair: "You're looking good. So courageous. Your body language. Truly, I watched you on TV." Blair responds: "Yeah, well. It's hard sometimes. Believe me. And you're doing pretty well yourself." Bush asks: "What me? I'm just ready to kick ass."

Chilcot and his team are known to have seen the play and it is understood that they requested further documents from the government shortly afterwards. At Chilcot's request, Helm confirmed that she knew shorthand, a clear sign that he was interested in the accuracy of her note-taking.

A Cabinet Office spokesman said discussions were continuing about the disclosure of the remaining documents. A spokeswoman for the Iraq inquiry declined to comment on the contents of the withheld documents.