In the history of British government, let alone British intelligence, there has never been a document to match the Butler report on weapons of mass destruction and the ingredients of the Blair Cabinet's decision to go to war with Iraq. Its effect is that of a lightning flash illuminating a spectrum of failures from human and political shortcomings in the Cabinet room to the unreliability of some of the human intelligence garnered inside Iraq.

In the history of British government, let alone British intelligence, there has never been a document to match the Butler report on weapons of mass destruction and the ingredients of the Blair Cabinet's decision to go to war with Iraq. Its effect is that of a lightning flash illuminating a spectrum of failures from human and political shortcomings in the Cabinet room to the unreliability of some of the human intelligence garnered inside Iraq.

Perhaps the most surprising revelation from its nearly 200 pages is the degree to which intelligence did not pave the road to war. From the spring of 2002, when the Blair government switched from a policy of containing Saddam to one of "enforcing Iraqi disarmament" - a shift which, in Butler's words, "was not based on any new development in the current intelligence picture on Iraq" - to the very eve of the war a year later, when the Attorney General's opinion that the imminent war was legal did not rest on new intelligence, the story is one of negatives.

In fact, in the spring of 2002, "there was no recent intelligence that would itself have given rise to a conclusion that Iraq was of more immediate concern than the activities of some other countries." And, as the arguments raged in Whitehall about the need for a specific United Nations resolution authorising the use of force against Iraq, the Attorney General, Lord Goldsmith, told his colleagues that "there would be no justification for the use of force against Iraq on grounds of self-defence against an imminent threat."

What about the famous warning in the Government's now notorious dossier of September 2002 that "Intelligence also indicates that chemical and biological munitions could be within military units and ready for firing within 20-45 minutes"? The Butler committee concluded that the original intelligence "report itself was vague and ambiguous", and its report reveals that the Secret Intelligence Service (SIS) subsequently acknowledged that "the validity of the intelligence report on which the 45-minute claim was based has come into question."

British intelligence prides itself on a core principle - a ruling tradition almost - that saw it through the Second World War and the Cold War. All raw intelligence is subjected to rigorous testing, validation and assessment, and is served up to its ministerial customers unvarnished, however inconvenient reading it might make. The Butler committee thinks the road to Baghdad caused that tradition to be tarnished. It wants it burnished and restored. Early indications from inside the British intelligence community suggest that this view is accepted, and it will be put right.

So, if the Blair government's decision to go to war was not evidence-based or intelligence-driven, do these shortcomings matter? Certainly they do. But in a second-order way. Intelligence garnered by SIS officers working in immensely difficult circumstances from but a handful of human sources somewhere in Iraq was bound to be precarious but partly revealing.

The first-order problem lies in Whitehall, in Number10, in the Prime Minister's Office and the Cabinet room. There is no graver decision for an open society than to go to war, especially if it is for the purposes of pre-emption rather than retaliation. And here Butler's analysis and critique verges on the devastating. The Joint Intelligence Committee of intelligence chiefs and senior officials may have been in error in allowing their customary caution and caveats to be removed as Number 10 helped them to shape the drafting of the September 2002 dossier, but their culpability does not match that which rings out of Butler's paragraph 610 on the failure to use the Cabinet system - the ultimate check and balance of British central government - properly:

"One inescapable consequence of this was to limit wider collective discussion and consideration by the Cabinet to the frequent but unscripted occasions when the Prime Minister, Foreign Secretary and Defence Secretary briefed the Cabinet orally. Excellent quality papers were written by officials, but these were not discussed in Cabinet or in Cabinet committee.

"Without papers circulated in advance, it remains possible but it is obviously much more difficult for members of the Cabinet outside the small circle directly involved to bring their political judgement and experience to bear on major decisions for which the Cabinet as a whole must carry responsibility.

"The absence of papers on the Cabinet agenda so that ministers could obtain briefings in advance from the Cabinet Office, their own departments or from the intelligence agencies plainly reduced their ability to prepare for such discussions... and lessened the support of the machinery of government for the collective responsibility of the Cabinet in the vital matter of war and peace."

The language is measured; the judgement is based on Butler's long experience of serving closely five prime ministers (including Tony Blair). Never has there been an indictment to match this of a systems failure at the heart of British government.

The writer is Attlee professor of contemporary British history at Queen Mary College, London. This article also appears in tomorrow's edition of 'The Tablet'