Tony Blair faced fresh accusations of a "cover up" today over his discredited claims about Saddam Hussein's weapons arsenal.

Brian Jones, a former nuclear and biological arms specialist at the Ministry of Defence, reignited the row over the Government's "dodgy" dossier on Iraq with new claims that Parliament was misled.

Dr Jones, the official at the Defence Intelligence Staff who was a key witness at the Hutton Inquiry, revealed that senior intelligence experts had rejected one of the most striking claims in the dossier.

While most attention has focused on the claim that Saddam could fire a WMD within "45 minutes", another key claim about the Iraqi regime speeding up production of biological and chemical agents was also deeply flawed, he said.

A highly secret MI6 report on the agents was included in the government report in September 2002 even though analysts considered it was "crap" and it had been rejected by them "within hours of seeing it", Dr Jones revealed in today's New Statesman.

The key piece of intelligence, dubbed "Report X", was officially rejected as coming from an unreliable source by July 2003, when MI6 formally withdrew it.

Mr Blair insists he did not know about the error until after the event, but Mr Jones points out that "any one of a number of officials in various government departments will have known and should have been alert to the danger of Parliament-being misled".

Dr Jones emerged as the "star" witness of the Hutton inquiry when it emerged he was the only official to formally object to intelligence caveats being left out of the dossier in the rush to its publication in the run-up to war.

He alleges that MI6 chief John Scarlett, chairman of the Joint Intelligence Committee at the time of the dossier's drafting, knew that defence intelligence experts had not approved "Report X".

"I am more convinced than ever that Report X was welcomed in September 2002, not as a particularly valuable piece of new intelligence but as a way to finesse a "sexed-up" dossier past the experts on WMD. The normal intelligence process of sceptical scrutiny was subverted," he said.

"I believe there were experienced intelligence professionals on the JIC who had seen Report X and understood it was not substantial. This means that the Government's claims [after the Butler Report on the dossier], that the intelligence process needed to be tightened.. .was part of a cover-up intended to blame intelligence rather than policy for the mistake that led us to war."

The Butler Report into the intelligence on Iraq revealed that the source of the last-minute report was discredited. The "sub source" who had allegedly passed on the information denied later to MI6 that he had said any such thing.

Lord Butler also found that former MI6 chief Richard Dearlove briefed Mr Blair personally on Report X. He told the Prime Minister that the source remained "unproven".

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