Whitehall mandarins launched an extraordinary 13-month effort to block the Iraq inquiry from publishing explosive memos between Tony Blair and George Bush, it emerged last night.

Cabinet Secretary Sir Jeremy Heywood’s attempts to keep the correspondence secret led to a major stand-off with Iraq inquiry chief Sir John Chilcot — and pushed back the publication of his report by a year.

Astonishingly, Sir John admitted he still did not know when the inquiry – which has already cost £9million – would finally be published, five-and-a-half years after it was launched.

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Sir John Chilcot told MPs it was 'really not possible to say' when he will publish his long-awaited report into reasons Britain went to war in 2003

Critics, referring to the TV comedy series Yes Minister, attacked the ‘Sir Humphrey Mafia’ for blocking the crucial evidence on how Britain entered the Iraq conflict in 2003.

MPs said that without the delays, the report would have been published by now and they criticised the ‘unnecessary suffering’ caused to the relatives of the servicemen and women who died.

Sir John, giving evidence to the Foreign Affairs Select Committee, described how the lengthy negotiations with the civil service over his request to publish classified material – including 30 notes between Mr Blair and President Bush – was ‘crucial’ in causing the delay.

He revealed that Sir Jeremy’s predecessor, Gus O’Donnell, ruled that the memos were ‘not disclosable’ as a group.

Negotiations with Sir Jeremy – nicknamed ‘Sir Cover-Up’ by his critics – on what to release began in August 2013 and did not conclude until September last year.

Sir John described the discussions as ‘agonising’ and ‘very long and difficult and challenging’ on both sides.

Critics have questioned whether Sir Jeremy was the right person to arbitrate on the documents as he was principal private secretary to Mr Blair in Downing Street from 1999 to 2003 – when the decisions to go to war were taken.

Sir John, himself a former civil servant, refused to criticise Sir Jeremy and denied his efforts amounted to ‘obstruction’.

Foreign Affairs Committee Sir Richard Ottaway intervened during Sir John's opening statement to bring it to a close

The former top civil servant was hauled before Parliament to explain why his report will not be published before the general election

CAREER DIPLOMAT WHO KEEPS DELAYING IRAQ WAR REPORT Sir John Chilcot was appointed by Gordon Brown Sir John Chilcot, the former Whitehall mandarin charged with heading the Iraq investigation, sat on the 'whitewash' Butler Inquiry into the use of intelligence in the run-up to the conflict. Sir John was one of the leading members of the inquiry which exonerated the Blair government for 'sexing up' the case for war. His appointment by Gordon Brown raised fears in Westminster of the latest in a long line of establishment cover-ups - following Butler and the Hutton report into the death of Dr David Kelly. A career diplomat, Sir John, is an intelligence expert who was principal private secretary to William Whitelaw during his time as Tory Home Secretary and also spent seven years as top civil servant in the Northern Ireland office. Advertisement

But he said it took a ‘considerable time’ before Sir Jeremy finally gave way and agreed to publish redacted versions.

‘The initial view taken by the previous Cabinet Secretary was that the notes – for example that Mr Blair had sent to President Bush – were not disclosable,’ said Sir John.

‘There was a strong convention that interchanges of that sort should not be disclosed in public.

‘As we went through, point by point, with the current Cabinet Secretary, it became increasingly clear that, on the balance of argument, he would agree that a certain passage or a certain point could be disclosed because of the essential nature of our inquiry, which related to the workings of central government.

'That came to a point where it was no longer possible to sustain a doctrine that these documents, as a category, could not be disclosed.’

Sir John said that members of his committee insisted on their publication after ‘retiring politicians and statesmen’ referred to issues in them in their memoirs.

Continuing on the subject of inquiry delays, Sir John pointed to departments being slow in producing documents he requested – saying the government ‘found it difficult to respond as quickly as we would have liked’.

His revelations angered MPs demanding the report be made public as soon as possible.

Former Tory shadow home secretary David Davis said: ‘We at last know that the Sir Humphrey Mafia tried every trick in the book to block the publication of the single most important piece of evidence, the correspondence between Blair and Bush, which will throw most light on the reasons we went into this disastrous war.’

And the Iraqi-born Tory MP Nadhim Zahawi said: ‘I’m convinced that if it hadn’t taken 13 months for these documents to be declassified we would have the report by now.’

He told the committee the failure to publish the report had created ‘uncertainty’ that was ‘clearly painful for the families’.

However, the revelation that Sir John dug in his heels during his standoff with Whitehall is likely to raise hopes his inquiry will be full and comprehensive when it reports.

He said he had a ‘duty to deliver the truth’ to the families of those who died and revealed the inquiry had examined some 150,000 documents.

‘The more we read the more lines of inquiry arose,’ he added.

It was ‘really not possible to say’ when the report would be published, as letters to those criticised in it, informing them of its conclusions and giving them the chance to respond, are still being sent out.

Tory backbencher David Davis has blamed the Civil Service, which has the final say over which documents relating to Iraq can be published, for the delay in the report's publication

TRIBUTES TO CHURCHILL BIOGRAPHER MARTIN GILBERT, WHO DIED AGED 78 Inquiry member Sir Martin Gilbert died last night Sir John Chilcot paid tribute to the renowned historian Sir Martin Gilbert, who died last night after a long battle with cancer. He said Sir Martin, a member of the Iraq report's panel since 2009, had helped the inquiry with his 'wisdom and insights'. The 78-year-old was the official biographer of Sir Winston Churchill and also one of the world's pre-eminent Holocaust historians. He was born in London in 1936 but was evacuated to Canada as a toddler when the Second World War took hold. Still a young child, he returned to Britain and was living near Oxford as the war drew to a close He authored over 80 books, many on Jewish topics, in a life that also saw a national service stint in the British Intelligence Corps. His works included classic histories of the first and second world wars, 'A History of the Jews of Europe During the Second World War,' and 'Jerusalem in the Twentieth Century.' Advertisement