American concern has been magnified by a series of intelligence and other reports warning that rapid advances in biotechnology could be exploited by terrorist groups seeking lethal bioweapons. "The technology for bio and chem is comparatively so much easier that that's where their efforts are concentrating," Bolton added.

In a study entitled The Darker Bioweapons Future, a CIA panel concluded that artificially engineered biological agents could prove "worse than any disease known to man". The report said: "The same science that may cure some of our worst diseases could create the world's most frightening weapons."

Experts believe Al-Qaeda still lacks the laboratory access and scientific skills to produce weapons. But some of the administration's scientific advisers have warned that the necessary technology is rapidly spreading. Some of it is even taught at undergraduate level.

"It seems likely that, over a period between a few months and a few years, broadly skilled individuals equipped with modest laboratory equipment can develop biological weapons," said Richard Danzig, a biowarfare consultant to the Pentagon.

Other US officials suspect Bin Laden may be planting his acolytes in university science departments in the same way that he sent the September 11 hijackers to US flying schools.

"This is a guy who thinks long-term," said one senior Washington source. "We have to learn to think like him."

Suspicion that Bin Laden is increasingly focusing on WMD was heightened by reports last October that he had sought permission from a well-known Saudi Arabian theologian for an attack that would cause mass American casualties.

Bin Laden's approach is said to have resulted in the publication of a religious decree entitled "Rules for the use of WMD against the infidels". It was issued by Sheikh Nasser bin Hamad al-Fahd, who is currently under arrest in Riyadh.

Not all scientists believe a group such as Al-Qaeda will ever master biowarfare technologies. The main fear is that a rogue scientist may be prepared to sell his expertise.

"The people that I worry about are the lone operators, the scientist who is disgruntled, deranged or just bought off," said Raymond Zilinskas, a Pentagon biowarfare consultant at the California-based Centre for Non-proliferation Studies.

"The probability of you or I dying from a terrorist bioweapon is smaller than our being eaten by a shark, but that is not to say we shouldn't worry about it."

American and British intelligence agencies have already confirmed Al-Qaeda's interest in chemical experiments. Training videos were found in Afghanistan indicating that enough cyanide had been produced to kill several dogs.

"Only a thin wall of terrorist ignorance and inexperience now protects us," said Danzig.