The crucial revelation that makes it impossible for John Scarlett to keep his job

The more detailed the study of the Butler report, the more worrying its revelations about the quality and assessment of the intelligence in the run-up to war. And, it has to be said, the more precarious the position of John Scarlett, the head of the Joint Intelligence Committee at the time and the new head of MI6 from this autumn.

The more detailed the study of the Butler report, the more worrying its revelations about the quality and assessment of the intelligence in the run-up to war. And, it has to be said, the more precarious the position of John Scarlett, the head of the Joint Intelligence Committee at the time and the new head of MI6 from this autumn.

The lack of caveats in the September dossier, Mr Scarlett's willingness to go along with Number 10's suggestions for presentation, and his readiness to accept "ownership" of a document that he knew to be at best less nuanced, and in parts more confident, than the assessments made by the committee he chaired, have already been well publicised.

But it is in the validity of the last-minute human sources who provided the really crucial information used to justify the assertions made about Saddam Hussein's possession of chemical and biological weapons that the most worrying points are now emerging. It was these last-minute additions that, in Butler's account, made all the difference to the tone of the dossier and the Prime Minister's later speeches to Parliament and the public.

Yet the information provided by these sources was deliberately kept from any of the analysts who might have checked it against other information and assessment.

The crucial paragraph is 405 of the report. In it, Lord Butler and his committee state that in mid-September 2002, a "report, described as being a new source on trial," was brought in which provided "significant assurance to those drafting the Government's dossier that active, current production of chemical and biological agent was taking place". A second report about a particular agent followed. "In July 2003, however," states the report, "SIS withdrew the two reports because the sourcing chain had by then been discredited."

The significance of this statement goes beyond what it says of the unsafeness of the dossier. It lies in the date at which the reports were withdrawn - July 2003. This was after the invasion but before Hutton took his evidence, and at the same time as the Commons Defence and Intelligence Committees were drawing up their assessments. Yet none of these reports contained a word about this crucial information, nor did either Sir Richard Dearlove, head of SIS, or John Scarlett refer to it in their evidence before the inquiries. Nor did Downing Street witnesses, if indeed they knew about it. How different would these reports have been if this information had been included. Would it have been Gavyn Davies, Greg Dyke and Andrew Gilligan who lost their jobs and not the heads of the intelligence community?

No one is claiming this as a smoking gun with which to discredit the Prime Minister or Lord Hutton. Nor are we, despite the accusations of government ministers, embarked on a witch-hunt for Dearlove or his successor. All of them could - almost certainly did - accept the new source in good faith and used its information at face value.

Yet the Prime Minister went on claiming that it was "beyond doubt" that Saddam Hussein was continuing to produce chemical and biological agents throughout 2003, and three successive investigations were allowed to proceed without the relevant information. If SIS withdrew the intelligence in July, one has to ask just when did they begin to have doubts, when did they tell the JIC, and why did they see fit not to tell any of the appropriate inquiries. This, after all, was no run-of-the-mill change in the evaluation of an ordinary piece of intelligence. This was the complete rejection of a central piece of a public document used to persuade Parliament and the people to support an invasion of a foreign country.

From the beginning, Tony Blair has been almost obsessional in his expression of public support for John Scarlett. In defiance of the ordinary laws of propriety, he went ahead with appointing him as the next head of MI6 before the Butler inquiry had reported. Butler's findings now make it impossible for him to continue with that appointment.