Blair did not go into detail, but a spokesman for the prime minister on Tuesday said the findings were part of an interim report produced several months ago by the Iraq Survey Group, which is hunting for weapons of mass destruction.
"The Iraq Survey Group has already found massive evidence of huge system of clandestine laboratories, workings by scientists, plans to develop long range ballistic missiles", Blair said in an interview with the British Forces Broadcasting Service.
He was responding to an interviewer who asked if captured former Iraqi leader Saddam Hussein might reveal details of his alleged banned weapons programme following his weekend arrest.
Blair replied: "There's obviously that possibility there but I think in any event we have got to carry on doing the work we are doing" in hunting for banned weapons.
"Frankly these things were not being developed unless they were developed for a purpose," the prime minister added.
"When a country with a leader like Saddam tries to hide what it's doing, in a large country like Iraq it's relatively easy to hide it."
"Frankly these things were not being developed unless they were developed for a purpose"
Prime Minister, UK