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."
The inquiry chairman also revealed 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."