There is one astounding judgment in yesterday's report from the Iraq Survey Group. That is that Iraq almost certainly did not have a serious chemicals weapons programme after 1991. US attacks and United Nations sanctions and inspections had made it impossible, it seems, for Iraq to rebuild what Desert Storm had destroyed.

That conclusion — although provisional — is damaging to Tony Blair. It is a profound contradiction of the September 2002 dossier, Blair's formal case for war, and in particular, of the claim that Saddam posed an imminent threat. It undermines that position more thoroughly than does the dispute about the now-notorious but narrow claim that Iraq could launch weapons in 45 minutes. It also implies a much wider failure of intelligence.

But if yesterday's report challenges the claim of imminent threat, it powerfully bolsters the case against Saddam Hussein which Blair made first, and should have retained as his best weapon: that the Iraqi leader was in systematic, large-scale breach of UN resolutions. The evidence is fragmented, but extensive: that Saddam set out to ignore those resolutions and had considerable success. With some help from his friends.

The section of the report that would no doubt be most explosive, but has been kept confidential, is the identities of the foreign companies and governments who systematically helped Saddam with his missile programme. In a damning final list, packed with detail apart from the identities of the offenders, the survey group spells out the extent of that help. UN sanctions may have worked on chemical weapons, but they failed comprehensively on missiles.

One optimistic note: the report mentions extensive help from many Iraqi scientists. Contrary to the popular image, the 1,200 members of the Iraq Survey Group are not forced to roam the desert blind, prodding at suspicious mounds of sand. The co-operation raises hope that the extent of the programme may eventually be determined.

You could not accuse this 12-page summary report of being "sexed up". It is technical, cautious, and extensively "caveated", as David Kay, its author, puts it. "It is far too early to reach any definitive conclusions and, in some areas, we may never reach that goal", he says.

It is worth remembering that Kay was not shy of headlines after the 1991 war when, as a UN inspector, he had the job of trying to work out how far Iraq’s nuclear ambitions had progressed. The answer was that they were on a much grander scale than anyone had imagined, but also that they were scattered, triplicated and fantastically inefficient.

Kay's energy in conveying the first part of that message, fuelled perhaps by the adrenalin of those discoveries and the standoffs with the Iraqis battling to conceal the programme, shocked the UN into realising that it had underestimated Saddam. Kay, a careful scientist, certainly relayed the second part of the message but it was less widely appreciated.

In retrospect, that distortion helped to exaggerate the perception of the threat Saddam then posed. It is arguable that this contributed to a fear in British and US intelligence agencies of underestimating Saddam again. Before this war, it may have encouraged a harder interpretation of available evidence than would otherwise have been the case.

This time, Kay has gone out of his way to emphasise the danger and difficulty of the work, and the imperfections of the conclusions. Iraqis have carefully destroyed much of the material that he sought. "It is important to keep in mind", he says, "that even the bulkiest materials we are searching for, in the quantities we would expect to find, can be concealed in spaces not much larger than a two-car garage".

In contrast, the September dossier chose to describe all the weapons unaccounted for in thousands of tonnes and litres, a more dramatic choice of units, but a misleading one, implying that the quantities would be hard to miss.

The sheer scale of the task, Kay points out, is why he may yet find chemical weapons, which Saddam generally stored unmarked among other weapons. The group has barely begun to search the "almost unbelievable scale of Iraq's conventional weapons armoury", he says, and has tackled only ten of its 130 ammunition storage points.

But "multiple sources" have already told him, he notes, that "Iraq did not have a large, ongoing, centrally controlled chemical weapons programme after 1991". Even in so cautious a report, he does allow himself this damning judgment: "Information found to date suggests that Iraq's large-scale capability to develop, produce and fill new CW munitions was reduced — if not entirely destroyed — during Operations Desert Storm and Desert Fox, 13 years of UN sanctions and UN inspections."

That does directly contradict the September dossier's assertion that "Iraq has a useable chemical and biological weapons capability . . . which has included recent production of chemical and biological agents."

Nor is there much sign of any sustained attempt to rebuild the nuclear programme after 1991, although scientists clearly retained the knowledge. This is constistent with what Britain and the US thought before the war.

On biological weapons, the report has found extensive small-scale research. That represents, Kay argues, an ability to "surge" production had Saddam chosen to do so.

On those points, there is little to help Blair and President Bush to defend themselves against the charge that Saddam did not pose as immediate a threat as they alleged.

But it would be entirely wrong to say that the Iraq Survey Group has found nothing. Kay cites half a dozen breaches of UN resolutions. That is still a powerful case against Saddam, even if it is not the one on which Blair and Bush finally chose to put most weight.

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