To wartime Britons, the chunky chocolate bar would have appeared manna from heaven in a time of rationing. Until, that is, it exploded as part of a Nazi plot to terrorise Britain.
German sabotage experts fashioned a range of bizarre weaponry, from high-explosive tinned plums to incendiaries disguised as frozen eggs, to be distributed by agents.
But MI5 files released at the National Archives in Kew, west London, show that British intelligence knew all about the scheme.
The weapons, such as the chocolate hand-grenade, designed to explode after the first row of chunks was broken off, were part of a campaign to undermine British resolve using sabotage and propaganda.
The bomb-makers were nothing if not ingenious. As well as packing explosives into cans of Smedley's English Grown Plums, the Nazis disguised plastic explosive as Wagon-Lits soap, hid detonators in torch batteries, and made exploding crucifixes, an incendiary Thermos flask and an exploding rat.
But the MI5 files show that the plot was a spectacular failure after the agents sent to distribute booby traps were all arrested or gave themselves up, and the propaganda incited more laughter than sedition.
Leaflets and fake newspapers prepared by Berlin for distribution across the United Kingdom were collected by MI5. A counterfeit copy of the Evening Standard, dated 17 February 1940, contained a lead story "uncovering" the unreported extent of RAF losses, a British plan to invade Canada and the groundwork for the Royal Family to leave the UK.
Most outlandish of all was a culinary section written by a fictitious French gastronome, M. Boulestin, suggesting that the "British breakfast problem" be solved by eating frogs. The article said the BBC would be broadcasting the recipes.
M. Boulestin wrote: "There are billions of frogs, of considerable size, hopping merrily round the British Isles. Their vitality should be harnessed."
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