The head of the civil service has refused to allow the official inquiry into the Iraq War to publish notes sent by Tony Blair to former US president George Bush.
Cabinet Secretary Sir Gus O'Donnell denied requests for exchanges between the former prime minister and Mr Bush about Iraq to be declassified and released.
Inquiry chairman Sir John Chilcot said: "The inquiry is disappointed that the Cabinet Secretary was not willing to accede to its request.
"This means that in a narrow but important area the inquiry may not always be able to publish as fully as it would wish the evidential basis for some of its comments and conclusions."
Sir John wrote to Sir Gus last month asking him to authorise the declassification of extracts from notes sent by Mr Blair to Mr Bush and records of discussions between the two leaders.
He highlighted the fact that Mr Bush and Mr Blair - as well as the former prime minister's chief of staff Jonathan Powell and communications chief Alastair Campbell - had revealed details of some of their talks in their recent memoirs, and said the inquiry's protocol on releasing documents supported disclosure.
Sir John said in his letter: "The inquiry regards it essential in order to fulfil its terms of reference, to be able to chronicle the sequencing of discussions on Iraq between the UK prime minister and the president of the United States.
"It seems to us that it is both contrary to the terms of the protocol and, in light of the disclosures in recent memoirs, unnecessary to prevent the inquiry from being able to do this."
He added: "In the inquiry's view it is essential, if it is to produce a reliable account, that it is able to quote extracts from the records of what the prime minister said to president Bush in their discussions on Iraq."
Sir Gus replied just before Christmas, writing: "My view is that the public interest is not best served by their release.
"I judge that their release would, or would be likely to, damage the UK's international relations."
He said the Cabinet Office attached "particular importance" to protecting the channel of communications between the British prime minister and the US president.
In a further letter to the Cabinet Secretary, Sir John said Mr Blair would face firm questioning about the content of his discussions with Mr Bush when he gives evidence to the inquiry for a second time on Friday.
He wrote: "Given Mr Blair's decision to disclose some of the content in (his memoirs) A Journey, the committee is likely to be disappointed if he is less forthcoming in his evidence to us.
"This approach is also likely to increase the length of the hearing."
Sir John added in a third letter to Sir Gus that the question of when and how Mr Blair made commitments to the US about Britain's involvement in military action against Iraq was "central" to the inquiry's considerations.
The inquiry chairman also revealed today that the committee recently took evidence in a closed session from David Pepper, the former head of the UK's signals intelligence agency GCHQ.
A Cabinet Office spokeswoman said: "All HMG (HM Government) documents have been made available to the inquiry.
"The issue is one of publication. Exchanges between the UK prime minister and the US president are particularly privileged channels of communication.
"The Cabinet Secretary is of the firm view that the public interest in publishing these letters is not outweighed by the harm to the UK's international relations that would likely be caused by his authorising their disclosure.
"This is in line with the published protocol.
"The majority of the inquiry's declassification requests have been met. But there are important public interest principles at stake. These are recognised in the protocol."
David Cameron was not consulted over whether to authorise publication of the documents, according to the Prime Minister's spokesman.
The Cabinet Secretary took the decision in line with a protocol set out at the beginning of the inquiry, he added.
"(Mr Cameron) had no role in that process," the spokesman said. "There is a protocol. That protocol sets out that in particular circumstances the Cabinet Secretary will make these judgments."
Long-standing procedures also prevented serving ministers from seeing documents relating to the work of previous administrations, he said.
Asked whether the Prime Minister was minded to change the protocol so that the documents could be published, the spokesman replied: "No."Reuse content