According to the letter, the crown jewels were buried under a small emergency exit from the castle. "What was so lovely was that the Queen had no knowledge of it. Telling her seemed strangely odd".
Giving her personal recollection, the queen also reveals how she had struggled with her coronation dress, which was embroidered in silk with pearls, and gold and silver thread.
A hole had to be dug and then covered to hide it from enemy bombers, while two chambers with steel doors were built to keep the stones safe.
A trapdoor used to access the secret area where the biscuit tin was kept still exists today.
She also said the golden ceremonial carriage used for her coronation was "horrible".
The operation was so secret that even the Queen didn't know what had happened until the filming of a new BBC documentary about the Coronation, which is set to be aired on BBC1 at 8 p.m. on Sunday, January 14. The Queen showed her wicked sense of humour by joking that you can't look down while wearing the Imperial State Crown - which weighs a staggering 1.28 kg.
Bruce watched the footage alongside Queen Elizabeth, but he does more than just reporting-he's also one of the queen's officers of arms, and he's such an expert in the semiotics (the language of symbols) of the nation that the Buckingham Palace often calls him for input and advice.
Speaking with the Imperial State Crown - worn when delivering her speech during the state opening of parliament - in front of her, the Queen said: "Fortunately, my father and I have about the same sort of shaped head".
The keepers of the royal things were a little anxious about all the important crowns and stuff going missing or getting bombed during the second world war, hence the most important elements of the Crown Jewels were taken from their position in the Tower of London and hidden away.
"But once you put it on, it stays", she said of the famous crown.
He added that Her Majesty had been aware that the jewels were hidden at Windsor by 1940, when the government was trying to hide stocks of water, but had no idea where they were buried - or that they were hiding in a biscuit tin.
The Queen with her crown jewels. "I mean, it just remains itself". "Because if you did your neck would break, it would fall off", she says.
"It's fun to see I think", she said. "They hadn't thought of that".
It is part of the Royal Collection season, in partnership between the BBC and Royal Collection Trust.
The revelations about the carriage were revealed in a new documentary about the amusing trials and tribulations of being head of state.