The Guardian's revelation that "Curveball", the renowned source of intelligence on Iraq's WMD, made it all up is yet another nail in the coffin of those who claim that the intelligence was clear about the alleged threat. Curveball's evidence that Iraq was secretly rebuilding a substantial biological weapons capacity was a key part of US and British claims that Iraq presented a growing and imminent threat.

Now that the truth about this propaganda has been revealed, we can expect that those who constructed it – Tony Blair, Dick Cheney et al – will now amend their usual arguments to suggest that they were innocently misled by evidence such as Curveball's. After all, if a defector claimed that there was a substantial bio-weapons programme, as "Curveball" did, how could they know that he was lying? Again, we will be confronted with the "not my fault!" excuse from those who manufactured the case for an avoidable war.

But once again, they are trying to mislead. Here's why.

As I learned in my work on Iraq's WMD in the late 90s and early 2000s, when I was Britain's Iraq expert at the UN security council and responsible for liaison with the weapons inspectors, intelligence on WMD is a confusing and complicated issue. There was a great deal of data, much of it contradictory, from an array of different sources – intercepts of communications, aerial and satellite imagery and "humint" from defectors or agents inside Iraq. Our task in the government was to try to make sense of all this, and interpret from the data a reasonably plausible and coherent picture of what was actually going on.

It is with sadness that I note that my most perceptive tutor in this complex art was David Kelly, the British weapons scientist who was then our foremost expert on biological weapons (BW) – but also skilled in the more comprehensive analysis of Iraq's WMD. Along with other weapons scientists, David would conduct detailed private seminars, organised by me, at the UK mission for other UN security council diplomats, to explain the evidence about Iraq's biological and chemical weapons, and missiles: what we knew, and what we didn't.

David was a highly-experienced BW scientist who had conducted scores of on-the-ground inspections in the former Soviet Union, as well as in Iraq. In his quiet, humble, yet authoritative way, he would patiently explain to diplomats like me, keen to make bold claims about complex data, that the reality of the intelligence was less clear. It presented many more questions than answers: in fact, all we could confidently speak of was what we did not know, rather than what we knew.

Given the complexity of the data, no single source could ever be taken as authoritative. And the least convincing sources – by their very nature – were defectors. We knew full well that, for very understandable reasons, defectors had a powerful incentive to exaggerate the nature of Iraq's development of WMD. They hated Saddam and wanted him gone. Long before Curveball, there were other defectors who made sometimes wild claims about Iraq's weapons programmes. I remember one report that suggested Iraq had armed its Scud missiles (none of which, in fact, existed, it later emerged) with nuclear warheads, ready to be launched at Israel and other targets. Defector intelligence was, therefore, lowest in the hierarchy of evidence; photographic or signals intercepts were, for obvious reasons, treated as more plausible.

Each piece of evidence, whatever its source, was first subjected to rigorous cross-checking before inclusion in overall analyses. All sources of intelligence suffered from particular deficits: Iraq knew that its signals were monitored and thus limited its communications traffic; it also hid any WMD activity under roofs in military and civilian sites, thereby limiting the value of overhead reconnaissance. So, all evidence had to be tested by the simple method of seeking corroboration from other sources. This method was used across Whitehall, and in the Ministry of Defence and the Cabinet Office in particular, and was the basis for the Joint Intelligence Committee assessments of the WMD threat, several of which I contributed to. In the years I worked on the subject (1997-2002), the picture produced by this method was very clear: there was no credible evidence of substantial stocks of WMD in Iraq.

And it was this method – clearly – that was abandoned in advance of the war. Instead of a careful cross-checking of evidence, reports that suited the story of an imminent Iraqi threat were picked out, polished and formed the basis of public claims like Colin Powell's presentation to the UN security council, or the No 10 dossier. This was exactly how a false case for war was constructed: not by the deliberate creation of a falsehood, but by willfully and secretly manipulating the evidence to exaggerate the importance of reports like Curveball's, and to ignore contradictory evidence. This was a subtle process, elaborated from report to report, in such a way that allowed officials themselves to believe that they were not deliberately lying – more editing, perhaps, or simplifying for public presentation.

David Kelly and I discussed very process – which he abhorred – a few weeks before he died. His revelation of it to a clumsy journalist, who called it "sexing up" (not a word that I think David would have used), was to trigger the events that led to his tragic death. Others of my former colleagues in the MOD and Foreign Office have freely admitted to me that this is precisely what took place. Yet, for all its subtlety and secrecy, we should name this process for what it was: the manufacture of a lie.