Tony Blair failed properly to discuss plans to join the US-led invasion of Iraq because he did not trust cabinet colleagues not to leak discussions, the UK's top civil servant said today.
And Cabinet Secretary Sir Gus O'Donnell told the Iraq Inquiry such an informal approach meant there were insufficient records to fully examine the decision to go to war.
Sir Gus levelled a number of criticisms at Mr Blair's so-called "sofa government" style as he was quizzed by the Iraq Inquiry, which is shortly to conclude its evidence gathering.
The then prime minister's distrust of colleagues meant he did not consider the cabinet the "safe space" it ought to be for the frank discussion of key policies, he said.
And failing to provide cabinet colleagues with the full legal advice of the attorney general on important decisions was contrary to the ministerial code, he said.
Lord Turnbull, who was cabinet secretary at the time, told the inquiry this week the cabinet was deprived of key papers outlining options and was "imprisoned" into backing the war.
He rejected assertions by Mr Blair, when he was recalled before the Inquiry last week, that his cabinet colleagues all "knew the score" and that there were frequent discussions of the issues.
Sir Gus, who was at that time the top official at the Treasury, said frank discussions within cabinet leading to an agreed line were the ideal formula for the best decision making.
But Mr Blair preferred to keep discussions to more exclusive groups, in part because distrust of colleagues meant he did not consider it the "safe space" it should ideally be, he said.
"That's one of the reasons why the prime minister was reluctant at times to take as many cabinet decisions as possible: he felt that they would become very public very quickly."
It meant there were gaps in the official records, he added.
"If you reduce the formality you do not have such good records. When you come to do an audit, as you are here, it is not as complete as any cabinet secretary would want it to be."
Asked about the failure to provide the cabinet with the full text of the then attorney general Lord Goldsmith's advice on the legality of the invasion to the cabinet, he said: "The ministerial code makes it clear that if there is a legal issue, the full text of the attorney general's opinion should be attached (to discussion papers)."
Summing up the lessons he thought should be learned from the Iraq build-up, he said: "Getting the cabinet and parliament operating ahead of military deployments on the right basis, on the basis of the right papers, and with the full written advice from the attorney general.
"I will encourage prime ministers to build cabinets where you are not afraid of challenge, where you build trust amongst people."
Sir Gus was not quizzed by the panel about his controversial decision to block the publication of secret messages sent by Mr Blair to then US president George Bush about the build-up to the Iraq invasion.
Despite expressing disappointment last week about the block on making extracts from the documents publicly available, inquiry chairman Sir John Chilcot began the session by indicating that it fell outside his remit to question the top civil servant about the matter.
The Inquiry was adjourned until next week, when the foreign secretary at the time of the invasion, Jack Straw, is among the last scheduled witnesses.