The revelation that the controversial claim is even weaker than ministers and officials have been saying will embarrass No 10, already reeling after the first week of the Hutton inquiry into the death of weapons expert David Kelly.
It came as the Hutton inquiry announced that Alastair Campbell, Downing Street's communications chief, will testify on Tuesday. Underlining the danger of the inquiry for the government, Lord Hutton has called virtually every member of the prime minister's inner circle.
The government has been under fire for including the allegation in a September 2002 dossier used to justify the war against Iraq.
The revelation that the 45 minute claim is second hand is contained in an internal Foreign Office document released by the Hutton inquiry. It had been thought the basis for the claim came from an Iraqi officer high in Saddam Hussein's command structure. In fact it came through an informant, who passed it on to MI6.
The document says the 45 minute claim "came from a reliable and established source, quoting a well-placed senior officer" - described by intelligence sources as a senior Iraqi officer still in Iraq.
The government has never admitted the key information was based on hearsay. On June 4, Tony Blair told the House of Commons: "It was alleged that the source for the 45 minute claim was an Iraqi defector of dubious reliability. He was not an Iraqi defector and he was an established and reliable source."
Adam Ingram, the armed forces minister, said of the claim on May 29: "That was said on the basis of security service information - a single source, it wasn't corroborated."
The irony is that the government launched a furious attack on the BBC for broadcasting allegations that the dossier was "sexed up" based on a single, anonymous, uncorroborated source. That source was Dr Kelly.
Mr Campbell told the foreign affairs select committee: "I find it incredible ... that people can report based on one single anonymous uncorroborated source."
In fact, the foundation for the government's claim was even shakier, according to the document: a single anonymous uncorroborated source quoting another single anonymous uncorroborated source.