Crumbs... royals hid gems in a biscuit tin during the Blitz

The tin was then buried in a deep hole beneath a sally port – an emergency exit from Windsor Castle.

The gems in the tin included the Black Prince’s Ruby from the Imperial State Crown, a BBC documentary reveals.

The Queen, who spent the war years at Windsor for safety, had known of the general story but was unaware of the details until they were revealed to her told by the programme’s presenter, royal commentator Alastair Bruce.

He said: “What was so lovely was that the Queen had no knowledge of it. Telling her seemed strangely odd.”

The full story emerged in a set of letters to Queen Mary, mother of George VI. They describe how a hole was dug in chalk earth, which had to be covered to hide it from enemy bombers, and two chambers with steel doors created. A trap door used to access the secret area where the tin box was kept still exists today.

The Queen recalls her 1953 coronation in tomorrow’s programme, and speaks of the difficulty of wearing the 2lb 13oz Imperial State Crown at the opening of parliament.

“You can’t look down to read the speech, you have to take the speech up. Because if you did your neck would break, it would fall off,” she says.

She also recalls being brought to a standstill when her coronation robes ran against the carpet pile in Westminster Abbey.