An outraged Winston Churchill accused civil servants of misleading him after discovering that his secret wartime bunker was not bomb-proof, according to a letter going on show for the first time in a new exhibition.
The document, written on September 13, 1940 by Patrick Duff, then Permanent Secretary at theOffice of Works, reveals how Churchill complained that Duff had "sold him a pup" and let him think that "this place is a real bomb-proof shelter".
He was referring to the underground rooms turned into a makeshift command centre, leaving the Cabinet far more exposed than Hitler's staff in his deeply embedded Berlin bunker.
In his letter to Sir Edward Bridges, the Cabinet Secretary, Duff confessed he was "indignant" at being accused by Churchill of misrepresenting the site's safety.
"On going through the plans of the rooms and explaining the stresses which they could stand up to and what they could not stand up to, the PM said I had 'sold him a pup' in letting him think this place is a real bomb-proof shelter, where it is nothing of the kind."
The Prime Minister calmed down once Duff explained that he had written to him on a previous occasion, clearly explaining the limits of the shelter.
The letter will be exhibited as part of the show, Undercover: Life in Churchill's Bunker, opening at the Churchill Museum and Cabinet War Rooms in London on August 27.
The letter is on loan from the National Archives at Kew, where it was discovered several months ago by the exhibition's curator, Cressida Finch.
"Although he was angry on learning that the War Rooms were not completely safe, he was determined not to leave central London and be seen as abandoning Londoners," Finch said.
"There is a direct comparison with Hitler, whose bunker in Berlin was 10m deep. Churchill's War Rooms, in effect a basement rather than a bunker, are only 3m below ground."
Later the same month, a bomb narrowly missed the War Rooms building and on October 14, bombs struck both the Treasury and Horse Guards Parade.
