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
Saddam's refusal to give up his alleged weapons of mass destruction was cited as one of the main reasons for Britain and the United States invading Iraq in March.
Blair has been US President George Bush's staunchest ally throughout the campaign, and Britain maintains 10,000 troops in Iraq occupying the oil-rich south of the country.
David Kay, the former UN weapons inspector who leads the Iraq Survey Group, said in October that evidence gathered by US teams suggested that Iraq had little or no capacity to produce chemical warfare agents because of damage inflicted by US air strikes and years of sanctions.
But Kay said the group had "begun to unravel a clandestine network of laboratories and facilities within the (Iraqi) security service apparatus" that was previously unknown and had never been declared to the United Nations.