Tipped with a sewing machine needle and finished with a tail made from a drinking straw, they looked more like a schoolboy's toy than a terrifying weapon. For Britain's wartime scientists, however, these tiny projectiles were the sharp end of a chilling project to secure victory over the Nazis by bombarding German troops with poisoned darts.

A secret file that details British research to develop the lethal anti-personnel darts, carrying a toxin likely to have been anthrax or ricin, casts rare light on the work that was carried out by the Allies during the Second World War into chemical and biological weapons that could be deployed against Hitler's forces.

The document, released at the National Archives in Kew, London, reveals how scientists at Porton Down in Wiltshire, the site of Britain's top secret weapons laboratory, worked between 1941 and 1944 to perfect the projectiles to ensure the maximum number of casualties and the quickest death for enemy soldiers.

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Entitled Research Into Use of Anthrax and Other Poisons for Biological Warfare, the report said the idea of using darts dated back to the First World War but the novelty of adding a poison, either coated on to a grooved point or injected through a hollow needle, meant that a viable weapon to cause "death or disablement" had been created.

A memo written in 1945 summarising the project said: "The use of poison enables a much lighter dart to be used, since a slight penetration without necessarily piercing a vital organ is all that is required to implant the poison ... It seems most unlikely that any first aid measure or medical treatment could be devised which would prevent the death of a man who has received a lethal dose."

The researchers, working in conjunction with Canadian colleagues, developed a dart weighing no more than four grams which could be loaded into bombs carrying 30,600 of the projectiles at a time. The researchers carried out multiple tests and calculations to work out the chances of hitting troops, ranging from 90 per cent for a soldier lying flat on open ground to just 17 per cent for one lying in a slit trench.

The consequences of being struck were dire. If a victim failed to pluck out each dart within 30 seconds, he was condemned to a grisly death. Detailing the effects of ricin, codenamed T1123, in tests on sheep and goats, one researcher reported: "The symptoms produced are: twitching of the muscles, profuse salivation and sweating, acute defecation, micturition and retching. The pulse becomes very slow and the blood pressure falls. The subject collapses and lies on its side with twitching muscles. Where the dose is lethal, death occurs in 30 minutes, usually preceded by convulsions."

Attempts by the scientists to perfect their projectile took on a darkly comical dimension when they approached Singer Sewing Machines Ltd, based in Bristol, to supply a variety of differently shaped needles without stating their purpose.

The request was met with bemusement by the company. In one letter sent in 1941, an executive wrote: "We are afraid we do not quite understand your requirements. From your remarks it would seem that the needles are required for some purpose other than sewing machines."

Despite the assertion of the researchers that their weapon was both more lethal and cheaper to make than conventional bullets, the darts never made it into mass production.

Noting that the projectiles were useless against any form of cover, a senior officer wrote them off as "highly uneconomical" and unlikely to cause mass casualties.