Washington is playing the lead role in delaying the publication of the long-awaited report into how Britain went to war with Iraq, The Independent has learnt.

Although the Cabinet Office has been under fire for stalling the progress of the four-year Iraq Inquiry by Sir John Chilcot, senior diplomatic sources in the US and Whitehall indicated that it is officials in the White House and the US Department of State who have refused to sanction any declassification of critical pre- and post-war communications between George W Bush and Tony Blair.

Without permission from the US government, David Cameron faces the politically embarrassing situation of having to block evidence, on Washington’s orders, from being included in the report of an expensive and lengthy British inquiry.

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Earlier this year, The Independent revealed that early drafts of the report challenged the official version of events leading up to the Iraq war, which saw Mr Blair send in 45,000 troops to overthrow Saddam Hussein’s regime.

The protected documents relating to the Bush-Blair exchanges are said to provide crucial evidence for already-written passages that are highly critical of the covert way in which Mr Blair committed British troops to the US-led invasion.

One high-placed diplomatic source said: “The US are highly possessive when documents relate to the presence of the President or anyone close to him. Tony Blair is involved in a dialogue in many of these documents, and naturally someone else is at the other end – the President. Therefore this is not Tony Blair’s or the UK Government’s property to disclose.”

The source was adamant that “Chilcot, or anyone in London, does not decide what documents relating to a US President are published”.

Last week, Chilcot sent Downing Street an update on his inquiry’s progress which explained his continuing inability to set a publication date. He described difficult discussions with the Government on the disclosure of material he wanted to include in his report, or publish alongside it.

He said that over the past six months, he had submitted requests that covered 200 cabinet-level discussions, a cache of notes from Mr Blair to Mr Bush, and more than 130 records of conversations between any two of Mr Blair, Gordon Brown and the White House. Mr Cameron was informed that the inquiry and the Cabinet Office had “not yet reached a final position” on the documents.

Although the Prime Minister told Chilcot in a letter last week that some documents needed to be “handled sensitively”, the Cabinet Office decoded the Prime Minister’s phrases yesterday, telling The Independent: “It is in the public’s interests that exchanges between the UK Prime Minister and the US President are privileged. The whole premise about withholding them [from publication] is to ensure that we do not prejudice our relations with the United States.”

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The Cabinet Secretary, Sir Jeremy Heywood, has been widely criticised as the senior civil servant responsible for blocking the delivery of material to the inquiry. Sir Menzies Campbell, who as the Liberal Democrats’ foreign-affairs spokesman was a high-profile opponent of the war, has described the delays as “intolerable”, adding: “The full story need[s] to be told.”

The former Foreign Secretary Lord Owen has called for Sir Jeremy to be stripped of his role in deciding which documents are released to the inquiry. However, the Cabinet Office said yesterday that Sir Jeremy was merely upholding a previous decision taken by his predecessor, Lord O’Donnell, which emphasised the importance of privacy in communications between Downing Street and the White House.

Chilcot, a former diplomat who previously investigated intelligence on Iraq’s supposed weapons of mass destruction as part of the Butler Review, heads an inquiry team that comprises Sir Roderic Lyne, the former UK ambassador to Russia; Sir Lawrence Freedman, the professor of war studies at King’s College London; and Baroness Prashar, a former member of Parliament’s Joint Committee on Human Rights.

Another member of the inquiry team, the historian Sir Martin Gilbert, has been ill and has had limited input into its recent deliberations.

The authors are facing difficult choices forced on them by Washington and the Cabinet Office’s desire not to upset the so-called “special relationship” between Britain and the US. They may deliver a neutered report in spring next year which would effectively absolve Mr Blair of any serious policy failures – because there would be no clear evidence contained in the report to back up such direct criticism. Another possibility is that the report will be so heavily redacted as to be rendered meaningless and hence a waste of almost £8m of British taxpayers’ money.

Since the Iraq Inquiry was launched in 2009 by the then Prime Minister, Gordon Brown, covert back-channel communications between the Cabinet Office and its counterparts in Washington have focused on the diplomatic convention that the disclosure of “privileged channels of communication” should remain at all times protected.

The final report is supposed to examine how the Blair government took decisions and what lessons can be learnt to “help ensure that if we face similar situations in future, the government of the day is best equipped to respond”.

Dr James Strong, a foreign-policy analyst at the London School of Economics, said: “All governments like to keep their secrets secret. The US is no exception. As its response to WikiLeaks suggested, the US defines a secret in terms of the type of document rather than the contents. So regardless of what these particular documents say, the US probably wouldn’t want them published, because governments normally keep private exchanges between leaders private.”

The US State Department declined to comment. Tonight, the Cabinet Office denied that the US had a veto on the issue, adding: “These issues are being worked through in good faith and with a view to reaching a position as rapidly as possible.”

In numbers: The Iraq war

179 British service personnel killed in Iraq

500,000 War related Iraqi deaths caused by the war to June 2011, according to 2013 PLOS Medicine study

1,569 Days since the Iraq Inquiry opened on 29 July 2009

35 Witnesses heard in private by Sir John Chilcot’s team

1,000,000+ Total length in words of the Iraq Inquiry report