Private memos sent between Tony Blair and George W. Bush in the months leading up to the Iraq invasion are to be kept secret after the head of the Civil Service consulted the former Prime Minister and refused to allow their publication.
The Cabinet Secretary, Sir Gus O'Donnell, vetoed attempts by the Iraq Inquiry to publish "key extracts" from the telegrams, which are regarded as highly significant by the inquiry team.
The inquiry chairman, Sir John Chilcot, described O'Donnell's decision as a serious blow to transparency.
One memo sent by Blair in 2002 is said to have reassured the US President that Britain would be "absolutely with you" should he take military action against Saddam Hussein.
The decision may severely hamper the inquiry team's ability to question the former Prime Minister about when he decided Britain would join the United States-led invasion.
Senior Whitehall sources confirmed that while committee members would be allowed to refer to the memos, they would not be able to discuss them in detail.
The Cabinet Office confirmed O'Donnell had consulted Blair before he decided not to authorise publication of the documents.
A spokeswoman said: "There is an established convention covering papers of a previous Administration whereby former ministers would normally be consulted before release of papers from their time in government."
A secret deal was struck in 2009 between the inquiry and Gordon Brown, then the Prime Minster, giving O'Donnell final veto over the release of sensitive documents.
Critics, including Deputy Prime Minister Nick Clegg, warned the protocols of the inquiry would mean the most damaging material would be kept secret.
The veto will put O'Donnell under scrutiny when he returns to answer questions in public before the inquiry next week. He is due to appear for three hours next Friday.
Speaking at the reopening of the public evidence sessions yesterday, Chilcot said he was disappointed 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."
Chilcot made a series of attempts to persuade O'Donnell to change his mind.