Britain considered dropping millions of poisoned darts on German troops in the final stages of the Second World War, secret files made public have revealed.

Created by British and Canadian scientists, the darts could have been packed into bombs and released from the air with the potential to kill or incapacitate anyone within 10,000 sq yds. Documents released by the National Archives under the Freedom of Information Act include letters and notes collected over four years that demonstrate how close the Government came to deploying the deadly darts.

The Singer sewing machine company was even approached to “unwittingly” provide needles for the weapon.

Designs show three different types of dart. One looks like a fountain pen, the others like a flat penknife. Records show that they were tested on sheep and goats in Canada to establish the effectiveness of dropping the projectiles from high and low altitudes.

A note from January 25, 1945, headed The Use of Poisoned Darts from the Air, said: “It is recommended that earnest consideration be given to possible utility of darts with a view to deciding whether development and exploration of this project should not be continued and intensified.”

Listed as “Top Secret”, it was written by an official from Porton Down, in Salisbury, which was then a government research centre for chemical and biological weapons. Scientists were working on the initiative with their counterparts at Suffield, a similar site in Canada.

The teams explored the most effective poison for the dart, comparing variations of urethane that caused death within 30 minutes with another substance, referred to only as “X”, that killed its victim within 24 hours.

Sheep and goats were again used in the tests, showing that the poison induced muscle twitching, salivation, sweating, defecation and retching. The pulse rate slowed and blood pressure fell as the animals collapsed and died.

Death would occur if a dart stayed in the body for more than 50 seconds. If it was taken out sooner, the victim might suffer a temporary collapse.

The delicate nature of designing a poison dart in secret is demonstrated in an exchange of letters between December 1941 and January 1942 in which Mr P. Fildes, an official at Porton, tries to procure a certain type of needle from the Singer company.

“We are afraid we do not quite understand your requirements,” the sewing firm wrote on December 24, 1941. “From your remarks it would seem that the needles are required for some other purpose, other than sewing machines. In any case, we should like to help you if at all possible.”

Mr Fildes responded: “It is a little difficult to explain what I want sewing machine needles for, but at any rate I can say that neither of the two samples sent were satisfactory. The knife-shaped point is definitely essential.”

British and Canadian scientists had different ideas on the best dart. The Canadians preferred a light one with a poisoned tip, while the British devised a heavier dart with the poison contained in a pellet behind the tip.

The two countries also explored different ways of firing the darts. One option was to pack 36,000 darts into a small bomb container which would be dropped at low altitude. The poisoned tips would travel at a fairly horizontal angle, making them most effective at killing soldiers standing up.

Another option was to pack 30,000 darts into cluster-bomb-style munitions. They would be dropped at high altitude, with the cluster projectile opening at about 3,000ft. The darts would fall vertically, with a good chance of hitting anyone lying, crouching or in a trench.

“The dart is a promising chemical weapon of a novel kind worthy of comprehensive investigation as soon as possible,” a Canadian report said.

Despite a compelling argument in favour of the darts, a serious impediment was that anyone in dense trees or a tent, or wearing a helmet, would be protected from the lethal tip. The documents noted that poisoned darts might be effective when first used but that the enemy would quickly learn to seek shelter.

“As a weapon they are highly uneconomical,” Colonel Stevenson, an officer, said on March 9, 1944. It is unclear why the weapon was rejected.

Mark Dunton, a contemporary history specialist at the National Archives, speculated that it could have been because Britain may have worried that its enemies would adopt the poisoned darts and use them on British troops.

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