One of Britain's top intelligence advisors yesterday strongly defended the controversial claim in the Government's dossier on Iraq that weapons of mass destruction could be launched in 45 minutes.

Former senior MI6 officer John Scarlett - now chairman of the Joint Intelligence Committee (JIC) responsible for drawing up the document - was making an unprecedented appearance before the Hutton inquiry into the death of weapons expert Dr David Kelly.

He insisted that there had been no interference from Number 10 in the intelligence judgements in the dossier on the threat from Iraq.

And the inquiry also heard that the intelligence agencies privately concluded that some Iraqi weapons could be deployed in as little as 20 minutes.

The inquiry was set up to investigate how Dr Kelly apparently came to take his own life after being identified as the source of a BBC report claiming the dossier had been "sexed-up" in the week before publication at the behest of Downing Street.

The inquiry was shown an e-mail from the Cabinet Office assessment staff, responsible for drafting the dossier, on September 11 - less than a fortnight before its publication - appealing to the agencies for more information.

It noted: "I appreciate everyone, us included, has been around some of these buoys before. But Number 10, through the chairman, want the document to be as strong as possible within the bounds of the available intelligence.

"This is therefore a last (!) call for any items of intelligence that the agencies think can and should be included."

However, Mr Scarlett told the inquiry that the request was "simply part of the work in progress".

He stressed that the document had simply been designed to show the intelligence assessment available to ministers, and was not intended to make the case for war. "In no sense, in my mind or in the mind of the JIC, was it a document designed to make a case for anything," he said.

Mr Scarlett insisted that the BBC report, by the Today programme journalist Andrew Gilligan, had been "completely untrue".

The inquiry also heard how the JIC had actually hardened its internal assessment of the controversial 45 minute claim in the dossier.

The original JIC assessment dated September 5, shown to the inquiry, noted: "Iraq has probably dispersed its special weapons, including its CBW (chemical-biological warfare) weapons.

"Intelligence also indicates that from forward deployed storage sites, chemical and biological weapons could be with military units within 45 minutes".

However an e-mail from the Defence Intelligence Staff the following day stated: "The intelligence reveals a maximum of 45 minutes. Average was 20 minutes. This could have important implications in the event of a conflict."

Mr Scarlett acknowledged that the information had come from a single source but said it was quoting a senior Iraqi military officer.

Although the claim that weapons could be fired within 20 minutes was not included in the published dossier, Mr Scarlett acknowledged that the language relating to the 45 minute claim had hardened during the drafting process.

Mr Scarlett accepted that Downing Street communications chief Alastair Campbell had asked if the language could be "tightened".

But Mr Scarlett said the Cabinet Office assessments staff had already decided to re-word the claim in order to correct a "slight muddle".

And he insisted that Mr Campbell had never sought to interfere in the intelligence judgements in the dossier and flatly denied reports of rows between them over the drafting process.