Adolf Hitler's bomb-makers coated explosive devices with a thin layer of rich dark chocolate and then packaged it in expensive-looking black and gold paper.
The Germans planned to use secret agents working in Britain to discreetly place the bars of chocolate -- branded as Peter's Chocolate -- among other luxury items taken on trays into the dining room used by the War Cabinet during the Second World War.
The lethal slabs of confection were packed with enough explosives to kill anyone within several metres.
But Hitler's plot was foiled by British spies who discovered that they were being made and tipped off one of MI5's most senior intelligence chiefs, Lord Victor Rothschild.
Lord Rothschild, a scientist in peacetime as well as a key member of the Rothschild banking family, immediately typed a letter to a talented illustrator seconded to his unit asking him to draw poster-size images of the chocolate to warn the public to be on the lookout for the bars.
His letter to the artist, Laurence Fish, is dated May 4, 1943 and was written from his secret bunker in Parliament Street, central London.
The letter, marked 'Secret', reads "Dear Fish, I wonder if you could do a drawing for me of an explosive slab of chocolate."
"We have received information that the enemy are using pound slabs of chocolate which are made of steel with a very thin covering of real chocolate," the Daily Mail quoted the letter as reading.
"Inside there is high explosive and some form of delay mechanism... When you break off a piece of chocolate at one end in the normal way, instead of it falling away, a piece of canvas is revealed stuck into the middle of the piece which has been broken off and a ticking into the middle of the remainder of the slab.
"When the piece of chocolate is pulled sharply, the canvas is also pulled and this initiates the mechanism. I enclose a very poor sketch done by somebody who has seen one of these. It is wrapped in the usual sort of black paper with gold lettering, the variety being PETERS.
"Would it be possible for you to do a drawing of this, one possibly with the paper half taken off revealing one end and another with the piece broken off showing the canvas.
"The text should indicate that this piece together with the attached canvas is pulled out sharply and that after a delay of seven seconds the bomb goes off," it added.
The letter was found by Fish's wife, journalist Jean Bray, as she sorted through his possessions following the artist's death, aged 89, in 2009.
A legendary Russian spy who foiled a Nazi plot to assassinate Winston Churchill, Josef Stalin and Franklin D. Roosevelt died in January, aged 87.
Gevork Andreyevich Vartanyan, codenamed Amir, ensured the safety of the three leaders by exposing a plot to kill them at the historic 1943 Tehran conference of the ''Big Three'' Allies.
He was just 19 at the time but he led a group of young Soviet agents to disrupt a German plot codenamed Operation Long Jump to wipe out the leaders of Britain, the USSR and the US.