Fierce opposition in Whitehall to the disclosure of key documents relating to the invasion of Iraq, notably records of discussions between Tony Blair and George Bush, has meant the Chilcot inquiry will not now be able to publish its report for well over a year.
Sir John Chilcot has made it clear in a letter to David Cameron that he and his fellow panel members are deeply frustrated by Whitehall's refusal to release papers, including those that reveal which ministers, legal advisers and officials were excluded from discussions on military action. The papers still kept secret include those relating to MI6 and the government's electronic eavesdropping centre, GCHQ.
The inquiry panel has seen the classified documents in dispute but is being prevented from publishing them.
In a letter to the prime minister released on Monday, Chilcot says there are unresolved disputes over "a number of particularly important categories of evidence" relating to "the discussions between the prime minister and heads of state or government of other nations" and "the treatment of discussions in the cabinet and cabinet committees".
Chilcot says Martin Gilbert, a historian and member of the inquiry panel, has been seriously ill since April, and has not been able to contribute to its work.
Chilcot refers to sharp exchanges he has had with the former cabinet secretary Lord O'Donnell over the refusal to disclose details of correspondence and conversations between Blair and Bush in the period leading up to the invasion. Their disclosure would serve to "illuminate Mr Blair's position at critical points" in the runup to war, Chilcot told O'Donnell last year.
He referred to passages in memoirs, including Blair's autobiography, A Journey; disclosures by Jonathan Powell, Blair's chief of staff; and the diaries of Alastair Campbell, his former head of communications. Those publications, and the refusal to disclose Blair's notes, Chilcot told O'Donnell last year, "leads to the position that individuals may disclose privileged information (without sanction) whilst a committee of privy counsellors established by a former prime minister to review the issues, cannot".
Campbell revealed his version of Blair's discussions with Bush in the latest volume of his diaries published last month.
O'Donnell told Chilcot that releasing Blair's notes would damage Britain's relations with the US and would not be in the public interest. "We have attached particular importance to protecting the privacy of the channel between the prime minister and president," he said.
Chilcot has emphasised that the protocols were "put in place to protect national security, international relations and the personal security of individuals. They are not there to prevent embarrassment."
O'Donnell, who consulted Blair about the documents, retired at the end of last year when he became a peer. He was succeeded by Sir Jeremy Heywood, who clearly shares O'Donnell's approach to the release of the documents.
Chilcot says in his letter that it is essential for the inquiry to establish "as accurately and reliably" as possible what happened and that some of the significant lessons to learn apply not only to Iraq. "Most have more general application to the conduct of government."
The final report, he adds, is likely to be more than 1m words. But the Chilcot panel will not be in a position to even complete the draft for a year. It will be in a position to "begin the 'Maxwellisation' process by the middle of next year", Chilcot told Cameron. Under this process, those whom the inquiry intends to criticise are given a copy of passages of the draft report to enable them to respond. The process derives from Companies Act investigations.
The inquiry initially suggested it would be completed in May 2011. It then said it could finish by autumn last year. It subsequently said it would not be published until this summer "at the earliest".
The inquiry held 18 months of public hearings between the end of 2009 and early 2011. A succession of witnesses, ranging from former cabinet secretaries to military commanders, sharply criticised the way Blair and his close advisers took key decisions without consulting senior ministers and the then attorney general, Lord Goldsmith.
Chilcot has said Blair's claim that MI6 established "beyond doubt" that Saddam Hussein had weapons of mass destruction was "not possible to make on the basis of intelligence".
Sir Michael Wood, the former senior legal adviser at the Foreign Office, said he considered resigning in protest against the war. Goldsmith said Blair's suggestion in January 2003 that Britain could attack Iraq without further UN backing had not been compatible with his legal advice.
Despite his refusal to release the Blair-Bush paper, O'Donnell said in evidence to the inquiry that the cabinet should have been told of Goldsmith's doubts about the legality of invading Iraq before Blair went to war.
"The ministerial code is very clear about the need, when the attorney general gives written advice, the full text of that advice should be attached [to cabinet papers]," O'Donnell said.
He said Blair did not believe cabinet was "a safe space" in which to debate going to war. That was one of the reasons why the then prime minister preferred informal meetings with no record taken.