Crucial doubts about Iraq's ability to produce chemical weapons were withheld from two inquiries which examined the Government's case for war.

Lord Hutton's investigation into the death of David Kelly and Parliament's Intelligence and Security Committee, which monitors the intelligence services, were not told that information which helped Tony Blair claim that Saddam Hussein posed a "serious and current" threat had already been discredited and withdrawn by MI6.

Three out of five key sources for the most sensational claims in the Government's September 2002 dossier on Iraqi weapons proved to be so untrustworthy that MI6 (the Secret Intelligence Service) officially withdrew their contributions. According to paragraph 405 of the Butler report, "in July 2003 ... SIS withdrew the two reports [about ongoing production of chemical weapons] because the sourcing chain had by then been discredited". The Hutton inquiry began taking evidence in August 2003.

Dr Brian Jones, a leading expert on chemical and biological weapons at the Defence Intelligence Staff who was not allowed to see the new intelligence, said last night: "This is very significant. Either the Prime Minister knew, when he gave his evidence to the Hutton inquiry, that the information from this source had been withdrawn in July 2003, in which case the question must be asked why didn't he mention it? Or, he was not told. In that case, surely, he must ask why he wasn't told, and whose decision was it not to tell him."

Lord Butler found that the two sources whose credibility is deemed to remain intact produced far less alarming reports about Iraq's supposed capabilities. The discredited information was the only material at the time suggesting that Saddam had "an ongoing chemical production facility", intelligence sources said.

The intelligence was withdrawn by MI6 in July 2003, after agents visiting Iraq found the secondary "sub-source", who had supposedly supplied their main source with intelligence, denied ever doing so.

Sir Menzies Campbell, foreign affairs spokesman for the Liberal Democrats, said: "Those who knew this information had been withdrawn - and who had previously provided it as being authentic and supportive of the dossier - had an obligation to tell others whose judgements were affected by it, as soon as it came to their notice that it couldn't be relied upon." He was "very surprised" that the ISC was not told and suggested that it might have reached a different conclusion.

On each of the three occasions where intelligence was withdrawn, MI6 told the JIC and Mr Scarlett of its decision. According to MI6, it was up to the JIC to pass this on to Downing Street. The Cabinet Office refused to comment yesterday on whether this was done, or when MI6 informed the JIC.

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