Winston Churchill accused civil servants of misleading him and "selling him a pup" after discovering his secret war-time bunker was not bomb-proof, according to a letter going on show for the first time in a new exhibition.

Winston Churchill accused civil servants of misleading him and "selling him a pup" after discovering his secret war-time bunker was not bomb-proof, according to a letter going on show for the first time in a new exhibition.

Written by Patrick Duff (permanent secretary at the Office of Works) to Sir Edward Bridges (Secretary to the Cabinet), the letter is on loan to Undercover: Life in Churchill's Bunker at the Churchill Museum and Cabinet War Rooms.

The letter recounts a meeting between Duff and Churchill on the morning of September 13 1940, during which the pair discussed the underground site of Second World War Cabinet operations.

Churchill claimed that Duff had "sold him a pup" by "letting him think that this place is a real bomb-proof shelter".

Duff confessed he was "indignant" at being accused by Churchill of misrepresenting the safety of the site.

"I replied with some emphasis that I had been at pain, literally for years past, when this place was originally taken for a War Room, and on every occasion since, when fresh essential personnel were put down there and required further accommodation, to represent that the place is not, and cannot be made, bomb-proof in any sense," he wrote.

Later in September 1940, a bomb narrowly missed the War Rooms building, making a crater near the Clive Steps at its north-west corner.

On October 14 bombs struck both the Treasury and Horse Guards Parade, causing blast damage to the kitchen, pantry and offices on the Treasury side of 10 Downing Street. This prompted Churchill to authorise the strengthening of the rooms with a concrete slab.

While the reinforcement made the site safer, it would not have been a match for the larger bombs at the end of the war, according to museum staff.

Phil Reed, director of the Churchill Museum and Cabinet War Rooms, said: "Since the Cabinet War Rooms opened to the public 20 years ago, there has been plenty of debate about how secure they were and whether they would have survived a direct hit during the Blitz.

"Certainly, several War Room veterans were unaware that the site was vulnerable throughout their period of service underground.

"This letter makes clear Churchill's surprise and indignation at being handed a potentially vulnerable bunker."

Exhibition curator Cressida Finch said: "This whole episode tells us a lot about Churchill's personal bravery.

"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.

"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. The contrast says a lot about the two men."

War Room typist Joy Hunter said: "When we were in the war rooms, thinking about the bombing and the buzz bombs and so on, I always felt completely safe, absolutely.

"It never occurred to me that the building could come down on top of us."