Two weeks ago, at lunch with the Queen, Lord Butler let slip that he was spending almost every waking minute writing his report into the intelligence that led Britain into war with Iraq. Even within Windsor Castle's protective walls, the former cabinet secretary gave little further away about what it will say, however.
In the five months since Tony Blair was forced to concede a formal inquiry into intelligence failures, there have been only the briefest of glimpses of Lord Butler's team at work. But a source close to the inquiry provides the first real indication of what is lurking in the wings for Mr Blair. It will, he says, be unsparing in its criticism of the "interface" between Downing Street and the intelligence services.
Having been cleared by Lord Hutton of the claim that Downing Street deliberately inserted false intelligence into a dossier on Iraqi weapons, Mr Blair must have thought he had weathered the storm. When David Kay quit the Iraq Survey Group, saying no stockpiles of weapons had been found, the clamour in the US for a proper inquiry became impossible for President Bush to resist, however. And once Mr Bush had sold the pass, the Prime Minister had to follow suit.
Still, he tried to limit its potential damage. "It should not be a rerun of the Hutton inquiry," Mr Blair said at the time. "We have dealt with the so-called sexing up of the dossier through three inquiries now. We do not need another inquiry into that. We do not need, in my view, an inquiry into the political decision to go to war. That's the matter for Parliament, government and the country in the end, but it's important we learn the intelligence lessons."
Lord Butler, however, has interpreted his terms of reference more widely than Mr Blair wanted, in particular with regard to examining "any discrepancies between the intelligence gathered, evaluated and used by the Government before the conflict".
The review can hardly ignore the multitude of "discrepancies" between how the claim that Iraq could deploy WMD within 45 minutes, for example, was "gathered, evaluated and used" in the September dossier authored by John Scarlett, head of the Joint Intelligence Committee (JIC).
One witness has told The Independent on Sunday that he expects the report to be particularly critical of how it was that Mr Blair came to think that the 45-minute claim related to Iraqi missiles instead of battlefield munitions. As this newspaper reported, a JIC assessment passed to Mr Blair shortly before the war was explicit in stating that the claim related to munitions, not missiles.
The review team is also known to be focusing on the dossier's claim that Saddam Hussein had tried to secure uranium from Africa. The intelligence services still insist they have credible evidence to back up the claim, referring to a visit to Niger by an Iraqi diplomat in 1999. But the diplomat told the IoS he had not been sent on a uranium-buying mission.
Lord Butler and his team have met senior intelligence officials in Washington, and are also said to have quizzed French and German officials about what intelligence they received and why they were so sceptical of British claims over Iraqi WMD.
What will be Downing Street's reaction to the report? Unlike with the Hutton inquiry, Mr Blair is likely to know its contents at least a week in advance, since it is a report to him, not an independent judicial inquiry. He will have plenty of time to prepare what is being dubbed a "non-apology apology".
"The vibes coming out of No 10 are that in some ways they welcome the opportunity to admit that they got some things wrong and that they have learned the lessons," said one senior figure.
A host of measures prepared in case Lord Hutton issued a critical report have been readied for the Butler report. Options include the JIC being chaired by a Foreign Office civil servant, as it was in the past, rather than by a member of the intelligence agencies.
More rigorous cabinet oversight of intelligence assessments is likely, as is a return to formal note-taking. Mr Blair's informal, unminuted "sofa diplomacy", revealed during the Hutton inquiry, is reported to have appalled Whitehall traditionalists.
Sir Andrew Turnbull, the Cabinet Secretary, has already told civil servants to keep more minutes, according to Professor Peter Hennessy, the leading expert on Whitehall.
Said it should not be a 'rerun of the Hutton inquiry' but will be dismayed to learn that Butler is focusing on the Iraq 2002 dossier. Faces renewed questions about why he believed the 45-minute claim related to missiles, not battlefield munitions
The other side of the 'interface' between Downing Street and the intelligence services. His triumphalism following the Hutton report is unlikely to be repeated next week
The man who authored the September dossier and now incoming head of MI6. Lord Hutton said that he may have 'subconsciously' been influenced by No 10. Lord Butler may be more blunt
Is said to be determined to avoid the charge that he has conducted a 'whitewash'. The head of a five-strong team is believed to have finished his report. It is expected to criticise the use of intelligence in the September 2002 dossier