Mr Blair cannot hope to put Iraq behind him until he owns up to his own faults in its presentation

In the current febrile atmosphere in British politics, it is easy to lose a sense of proportion

In the current febrile atmosphere in British politics, it is easy to lose a sense of proportion. A vicious clash in the Commons, a rumoured resignation threat or a leaked memo can seem more explosive than they really are. But today's publication of the Butler report into British intelligence is a moment of genuinely profound political importance.

The Hutton report into the death of a single individual was largely a diversion. Lord Butler's remit gets to the crux of the matter. For more than a year, the two key questions relating to the origins of the war in Iraq have remained unanswered: Why was the intelligence so wrong? And why did Tony Blair believe it with such an apparent passion and present it to the public without any qualifications? Lord Butler's report and Mr Blair's response to it might provide the answers.

We already know much of the information presented to Lord Butler. The report by the Commons' Intelligence Committee criticised how intelligence relating to weapons that could be deployed within 45 minutes was used. The Committee also reported that Mr Blair received intelligence warning that a war would increase the threat posed by terrorists. Mr Blair chose not to publish this advice.

Separately, the Hutton Inquiry revealed that senior figures in Mr Blair's office raised doubts about whether the intelligence justified the claim that Saddam Hussein posed a current and growing threat. The inquiry also confirmed that the Joint Intelligence Committee worked closely with Downing Street in compiling the dossier on Saddam's weapons. We know too that much of the original intelligence was tentative - and yet Mr Blair declared repeatedly that he had no doubt that Saddam possessed WMD. Britain went to war to disarm Saddam at a time when he seems to have had no arms.

Lord Butler is unlikely to deliver a killer blow. Even so, he cannot avoid making severe criticisms of intelligence-gathering and the way that intelligence was used by Downing Street in the build-up to war. For the first time, Mr Blair must address in detail the gap between the speculative intelligence and the alarmist way in which he presented it. He cannot get away with false arguments as he did last week when he was questioned by a committee of MPs. He said then that: "While I must accept that the weapons may not be found, that does not mean Saddam posed no threat at all." Few are arguing that he posed no threat. The key point is that many questioned at the time whether Saddam posed a threat to the extent that Mr Blair insisted he did. They have been proved right. Mr Blair, who took this country to war, has been proved wrong. Nor can Mr Blair escape, as he has tried to do in the past, by posing the question: "What would you have done in the light of the intelligence?" Other world leaders and his former foreign secretary recognised the flaws in the intelligence in advance of the war.

Today will be an important parliamentary occasion. Mr Blair is a skilful performer in the Commons. He is capable of making the most banal excuse seem profound and convincing. It will be the responsibility of Michael Howard and Labour opponents of the war to probe him forensically.

For the intelligence agencies, hugely important lessons are there to be learnt. The world is more dependent than ever on accurate intelligence - and the United States and Britain are not very good at delivering it. The report last week by the Senate committee into US intelligence was devastating in its castigation of the CIA and the "group think" that led it to assume that Saddam possessed WMD. Lord Butler will only open himself to ridicule if he does not make at least a hint of the same criticisms in his.

Mr Blair seeks to move on from the origins of the war in Iraq. He cannot do so, and will not deserve to do so until he and senior intelligence officials acknowledge their errors - and demonstrate convincingly that they will learn from them.