Experts from Harvard Medical School have found six chemicals which they believe could stop a toxin called "lethal factor" getting into cells. In the journal Nature Structural Biology, they say a drug could be more useful than mass vaccination.

The threat from a biological weapon containing anthrax is unknown, although estimates suggest a kilogram (2.2 pounds) of anthrax spores released over a major city would claim thousands of lives, regardless of whether antibiotics were administered promptly.

The bacterium is dangerous because it produces a variety of toxins which, together, can penetrate human cells and kill them. There is currently no way of tackling these toxins apart from early treatment with antibiotics.

The Harvard researchers, alongside others from Beth Israel Deaconess Medical Center in Boston, used a new technique to screen huge numbers of tiny protein fragments to locate some which could latch on to the toxin and interfere with its normal working.

They found six possible candidates, which laboratory tests showed appeared to have the desired affect on anthrax toxin.