It was an immaculately dressed Sir Richard Dearlove who hurried through the door of 10 Downing Street to meet Tony Blair on September 12, 2002. The Iraq crisis was building and the chief of MI6, the Secret Intelligence Service, had vital news for the prime minister.

Senior officers in MI6’s Middle East directorate had told him that a “new source” had emerged in Iraq and delivered a bombshell. The shadowy informant was claiming that Saddam Hussein was “accelerating” the production of biological and chemical agents and that new laboratories were spreading out across the country.

Within hours of receiving initial reports of this intelligence two days previously, Dearlove had alerted Sir David Manning, Blair’s foreign affairs adviser. As opposition to the war rose, the new information could prove pivotal for the government’s position. Now Dearlove was at No 10 to brief Blair in person.

The spymaster explained in characteristically measured tones how the new source appeared to have good “access” to Saddam’s inner circle. The informer had indicated to his MI6 handler that he would be able to provide substantial further intelligence in the near future.

Dearlove hoped that the informant would become an important asset, although he emphasised that the source was still “on trial”. The agent’s credibility, he added prophetically, remained “unproven”.

To Blair’s closest advisers it was like the cavalry coming to the rescue. Jonathan Powell, his chief of staff, and Alastair Campbell, his communications director, were in the midst of reviewing the now notorious dossier on Saddam’s weapons of mass destruction. They and John Scarlett, head of the joint intelligence committee (JIC) who had formal responsibility for the dossier, were all struggling to find convincing evidence of the existence of Iraq’s weapons, let alone anything which showed that the threat was growing worse.

For the Downing Street insiders it was no time to worry about an agent’s “credibility”.

Twelve days later the dossier was signed off by Scarlett and published. In its foreword Blair claimed that Saddam “is continuing to develop” weapons of mass destruction and in the House of Commons he declared that the intelligence was “extensive, detailed and authoritative”. The truth was that the intelligence was patchy, inconsistent and hedged with caveats.

Dearlove’s “new source” was, in fact, deeply flawed. The information had come via an MI6 agent in Iraq from a “sub-source” in the country — and that person, when tracked down after the invasion, denied ever having provided the sort of intelligence that had been relayed to Blair.

Worse, this was far from the only instance of “intelligence” being ephemeral. As an official report into the affair published last week revealed, MI6 only ever had a handful of key sources on Iraq’s weapons. Three are now discredited; two were always largely irrelevant.

Far from Iraq being a “real” and “current” threat, Lord Butler, author of the report, concluded that it “did not have significant — if any — stocks of chemical or biological weapons in a state fit for deployment”.

Britain had been persuaded into war on thin, patchy “intelligence” that proved to be no more believable than a James Bond plot. The chairman of the JIC and the head of MI6 knew the limitations. But Blair never told the country.

Behind the mandarin language of Butler’s report, the bottom line is that Blair took the country to war on a false prospectus. There can be no more serious failure in politics. How could it ever have happened?

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