Churchill felt he was 'sold a pup' of a bomb shelter, letter reveals

While Hitler had a proper shelter, Britain's Cabinet made do with a vulnerable cellar

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 13 September 1940 by Patrick Duff, then permanent secretary at the Office 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 commande 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. "The Prime Minister called for someone from this office to go to No 10 this morning to talk to him about the central War Room, and I thought it would be well that I should go myself," wrote Duff. "I am glad I did because, 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 civil servant added.

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 27 August. 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.

Churchill's fury actually revealed his "personal bravery", Ms Finch said. "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," she added. "Also, there is a direct comparison with Hitler, whose bunker in Berlin was 33ft deep. Churchill's War Rooms, in effect a basement rather than a bunker, are only 10ft below ground."

Terry Charman, a senior historian at the Imperial War Museum, said Britain's wartime leader was known for his occasional outbursts, adding: "He was a person who had explosions of anger but they didn't last long. A lot of secretaries he worked with said he'd shout at them in moments of frustration but then became kind and gentle."

Churchill's fears about the bunker's safety were well founded. Later in the same month, September 1940, a bomb narrowly missed the War Rooms building, leaving a crater near the Clive Steps at its north-west corner. On 14 October, bombs struck both the Treasury and Horse Guards Parade. Shortly afterwards, Churchill authorised the strengthening of the War Rooms, with a large concrete slab over part of it, though this would still not have withstood the impact of the larger bombs in use by the Germans by the end of the war.