The late government weapons inspector Dr David Kelly said it could take Saddam Hussein "days or weeks" to mobilise weapons of mass destruction.
The weapons expert said Iraq was a threat to its neighbours
His view was at odds with the claim in the government's Iraq dossier that "military planning allows for some of the WMD to be ready within 45 minutes".
The interview was recorded for Panorama in October 2002, a month after the prime minister presented the dossier to parliament, but never broadcast.
In the interview Dr Kelly also made it clear that he did regard Saddam as an "immediate threat".
Describing Iraq's weapons, Dr Kelly told Panorama: "Even if they're not actually filled and deployed today, the capability exists to get them filled and deployed within a matter of days and weeks."
He also said Saddam Hussein's biological weapons programme posed a "real threat" to neighbouring countries.
Dr Kelly said: "We're talking about Iran and Israel, and certainly he can use those weapons against them and you don't need a vast stockpile to have a tremendous military effect."
Whilst Dr Kelly was publicly generally supportive of the Blair government's tough stance on Iraq, parts of the interview are consistent with his more nuanced criticisms of the dossier, which he made in private to journalists and others.
For example, Dr Kelly said that whilst he believed Saddam Hussein still had a WMD capability, he did not pose the threat that he did prior to the first Gulf War.
"Iraq's intrinsic capability has been reduced since 1990/ 91," the former UN weapons inspector said.
His interview also points to sizeable gaps in British intelligence's knowledge of Saddam Hussein's WMD programme.
When asked about the means of delivery for Iraq's biological weapons, he said: "The actual form, we really don't know.
"He would have been planning to develop them and have far better and far more effective systems, and those we are completely unsighted of, and we're unsighted as to whether that work has continued since 1991 to this very day."
Dr Kelly also appears to have been at odds with the suggestion that Saddam Hussein was likely to use WMD even if he was not attacked first.
Asked if the deposed leader would use his WMD, Dr Kelly's answer implied that he believed he would have only used them defensively.
Dr Kelly said: "I think he would use them.
"Of course, what is more difficult to answer is how and under what circumstances he would use them.
"I think some people would consider that when the chips are really down, and he's fighting his last battle, that he may be prepared to use them.
"I think he would be reluctant to use them in the build up to war - in the transition to war - because he knows what the response would be.
"It would be utterly devastating for him."
Dr Kelly said Saddam Hussein might use his weapons
Originally, the penultimate draft of the Iraq dossier had also suggested Saddam Hussein was likely to use WMD only in self defence.
However, the Hutton Inquiry was told that a paragraph was re-written at the request of Mr Blair's Chief of Staff, Jonathan Powell, who e-mailed Alastair Campbell and others in Number 10 that it could be "a bit of a problem".
Mr Powell said it bolstered the argument that Saddam Hussein only posed a WMD threat if he was attacked first.
John Scarlett, Chairman of the Joint Intelligence Committee who was in charge of putting the dossier together, agreed to the change shortly before it was due to go to the printers.
He told Lord Hutton the change was justified by another intelligence assessment.
Dr Kelly became a key figure in the row between the government and the BBC over claims that Downing Street "sexed up" a dossier on Iraq's weapons capability.
He was thrust into the spotlight after being identified in newspapers as the man the government believed was the source for Andrew Gilligan's report on the Today programme.
The piece included the allegation that the government had put intelligence into the dossier which it probably knew to be wrong at the behest of Number 10 in order sex up the dossier.
Dr Kelly told a Foreign Affairs Select Committee hearing into the Iraq dossier that he did not think he was Gilligan's source.
After his apparent suicide, the BBC acknowledged that he was.
Panorama: A fight to the death was broadcast on BBC One on Wednesday, 21 January 2004 at 2030 GMT