British authorities feared the Nazis would use poison gas as part of an attempt to invade the UK, newly disclosed files show.
Intelligence suggested Germany had stockpiled large amounts of chemical weapons and carried out tests using anthrax-infected mortar shells and a foot-and-mouth disease spray.
An unconfirmed report even suggested that the Nazis had developed a gas which could stop car engines from working, previously secret documents released by the National Archives reveal.
In January 1941, when a German invasion was feared to be imminent, British military intelligence concluded that Hitler's forces were likely to use poison gas against the UK.
"If Germany attempts to invade this country she will be undertaking a most hazardous operation for which the prize will be world domination," officers wrote in a report. "Into that operation she is certain to put all her strength, and if she considers that the use of gas will increase her chance of success, she will not hesitate to use it."
The briefing noted that Germany's highly developed chemical industry had already created large stocks of mustard and phosgene gas, which were reportedly being moved from factories in readiness for a possible invasion.
"There is likely to be no bar to the use of gas by Germany on account of the lack of stocks or of trained personnel or weapons for producing it," stated the report drawn up by MI14, the specialist German intelligence branch of the War Office.
British officials concluded that the Nazis were not likely to use poison gas on civilians in general, but might do so to provoke a panicked evacuation of coastal areas ahead of an invasion.
Another intelligence report, from February 1941, suggested that the Nazis were preparing to accuse the UK of launching gas attacks to justify their own use of chemical weapons.
"A foreign diplomat indicated that the German authorities are fostering a belief in the intention of the British to use gas in their raids against Germany, and concludes that this propaganda is intended to prepare German public opinion for the German use of gas against the United Kingdom," it said.