On February 5, 2003, Colin Powell, one of the most respected members of President Bush’s war Cabinet, sat in the heart of the UN Security Council, surrounded by the world’s diplomats and in front of a global television audience.

The former US Secretary of State – with George Tenet, the CIA Director, behind him – presented a bone-chill-ing account of Iraqi biological weapons sites. At one point he waved a model vial of anthrax (it was talcum powder) to demonstrate the danger Saddam Hussein posed to the world.

He spoke of “an Iraqi chemical engineer who supervised” one of Saddam’s mobile biological weapons labs. “He actually was present during the biological agent production runs,” Mr Powell said. “He was also at the site when an accident occurred in 1998. Twelve technicians died from exposure to biological agents.”

This brave engineer, Mr Powell added, was one of those “who have risked their lives to let the world know what Saddam Hussein is really up to”. The man – the primary intelligence source justifying the invasion of Iraq that took place a month after Mr Powell’s address – was a chain-smok-ing, alcoholic, confidence trickster and Iraqi defector called Rafid Ahmed Alwan, codenamed “Curveball”.

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No US intelligence official had met him. His German handlers had warned CIA officials that he was “crazy”, unreliable and probably a “fabricator”. Yet his first-hand claims about Saddam’s biological weapons labs – all proved to be lies – formed the heart of the US intelligence community’s pivotal October 2002 National Intelligence Estimate presented to President Bush. Based solely on Curveball’s unverified fantasies, it stated with “high confidence” that Saddam “has” an illicit biological and chemical weapons programme.

Curveball’s story is also at the heart of the manifold and breathtaking failures of US spy agencies in the run-up to war, from how intelligence was gathered, analysed and eventually presented to an Administration determined to topple Saddam.

In a devastating 601-page report released in March 2005, by a commission established by President Bush to investigate prewar intelligence blunders, the almost-exclusive reliance on Curveball and abject failure to check his claims were cited as the primary reasons that the CIA and other agencies were “dead wrong” in nearly all their prewar assessments.

“Worse than having no human sources,” the commission said, “is being seduced by a human source who is telling lies.”

Curveball, a former taxi driver and chemical engineer, defected to Germany in 1999 in search of political asylum – and a Mercedes – and was willing to say anything to get them. He initially convinced his German handlers, with his impressive and detailed knowledge of biological weapons programmes (much of it gleaned off the internet) that Iraq had seven mobile weapons labs. German intelligence passed transcripts on to their US counterparts but, because of simmering Cold War rivalry, never allowed Curveball to be interviewed by American officials.

By the time of the National Intelligence Estimate, the Germans had grave doubts about Curveball. Several CIA officials testified to the commission that after they had been told by German agents that Curveball had suffered a breakdown – and was probably a liar – they strongly voiced their concerns to Mr Tenet and his deputy, John McLaughlin. Tyler Drumheller, a veteran CIA officer, says that he urged Mr Tenet not to include Curveball’s claims in Mr Powell’s UN speech. Mr Tenet and Mr McLaughlin say that they received no such warnings.

Nevertheless, even a cursory investigation of Curveball’s claims would have revealed them to be bogus. The deadly biological lab incident that he said he witnessed took place in 1998, when he “was not even in Iraq at that time” – a fact confirmed by his travel records, the commission found. Meanwhile, the allegation that Iraq sought to acquire uranium from Niger was based on “transparently forged documents” purporting to show a contract between the two countries, the commission said. There were “flaws in the letterhead, forged signatures, misspelled words, incorrect titles for individuals and government entities”. The fake contract referred to an alleged meeting “that took place on ‘Wednesday, July 7, 2000’, even though July 7, 2000 was a Friday”, the report said.

According to the investigative journalist Bob Woodward, Mr Tenet assured Mr Bush at a meeting on December 12, 2002, that the evidence against Saddam was a “slam-dunk case”. Mr Tenet said that his words were taken out context; he was saying that persuading the US public to back the war would be a “slam-dunk”.

Mr Bush and Dick Cheney, the Vice-President, who were convinced that Saddam possessed illicit weapons, needed little persuading. “Simply stated, there is no doubt that Saddam Hussein now has weapons of mass destruction. There is no doubt he is amassing them to use against our friends, against our allies and against us,” Mr Cheney declared on August 26, 2002, to the national convention of the Veterans of Foreign Wars. He said in the same address that Saddam had reconstituted his nuclear weapons programme, a claim with no credible intelligence to back it up.

Across the Atlantic, Britain faced a similar intelligence failure. The Government’s intelligence dossier, published in September 2002, provided the “evidence” of Iraq’s weapons of mass destruction and was used by Tony Blair to convince Parliament that Saddam had to be confronted. The claims proved unfounded and were a disaster for politicians and spy chiefs alike.

It cannot be overestimated how big a shock it was for the intelligence community to discover that the weapons of mass destruction did not exist. Indeed, such was the disbelief that they had got it wrong that some senior figures remained confident for a long time that eventually the weapons would be uncovered. In the event the Iraq Survey Group, a team of more than 1,000 Americans, Britons and Australians given the job of finding Saddam’s WMDs, turned up with two chemical weapons shells dating back to the Iran-Iraq War after scouring the country for 18 months. Since then Britain’s intelligence services have overhauled the way they recruit and run agents. There is now a three-lay-ered system that analyses and checks the veracity and value of intelligence.

Similarly, in the US, the searing experience of Curveball has led to a far more rigorous and meticulous vetting of sources. “You would not now have Curveball getting the prominence he did,” said Bruce Riedel, a former CIA veteran and senior fellow at the Brookings Institution think-tank.

Mr Tenet left the CIA in July 2004. Curveball lives in hiding, protected by the authorities, in Germany – probably still without a Mercedes.

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