Dr David Kelly believed Saddam Hussein possessed weapons of mass destruction and might have used them, it was revealed today.
But government weapons expert Dr Kelly said it could take "days or weeks" to deploy them and suggested the Iraqi leader would only do it if attacked.
Dr Kelly was found dead at a beauty spot after being identified as the source of BBC claims that Downing Street "sexed up" intelligence on Iraq.
Tony Blair presented a dossier to MPs saying "military planning allows for some of the WMD to be ready within 45 minutes".
Speaking to BBC's Panorama a month later, in October 2002, Dr Kelly said the Iraqi dictator did pose an "immediate threat".
On WMD, he said: "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."
At Westminster MPs questioned why the BBC had not produced the previously undisclosed interview to the Hutton inquiry or before.
Labour's Chris Bryant questioned whether the Corporation's governors could have seen it before rushing to defend correspondent Andrew Gilligan's accusations about Number 10.
"It seems very curious that this footage has only just come to light now," Mr Bryant said.
"I would have thought it could have played a helpful role both in the BBC governors' assessment of the rights and wrongs of Andrew Gilligan's report and the subsequent Hutton inquiry.
"I wonder whether the governors knew of the existence of this."
Tory MP Robert Jackson added: "If this interview appears to have been relevant to the inquiry, the BBC has questions to answer."
Dr Kelly was Britain's foremost expert on Saddam's biological weapons and says in tonight's progrramme that they posed a "real threat" to neighbouring countries.
"We are 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," he said.
His remarks, which have not been made public before, were posted on the BBC's website ahead of a special edition of Panorama on BBC1 at 8.30pm tonight.
It is being broadcast a week before Lord Hutton publishes the findings of his inquiry into the circumstances surrounding the death of Dr Kelly.
Saddam posed a much lesser threat than he had prior to the first Gulf War, Dr Kelly told Panorama.
"Iraq's intrinsic capability has been reduced since 1990/1," the former UN weapons inspector said.
But British intelligence had sizeable gaps in its knowledge of Iraq's WMD, he revealed.
Asked about how Iraq might launch biological agents, he said: "The actual form, we don't really know.
"He would have been planning to develop them and have far better and fare 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 suggested Saddam might have used his arsenal only if attacked.
"I think he would use them. Of course, what is more difficult to answer is how and under what circumstances he would use them," he said.
"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."
The BBC said later in a statement: "A transcript was disclosed to Lord Hutton while the Inquiry was sitting."Reuse content