A collection of priceless gems from the Crown Jewels were kept hidden underground in a biscuit tin at Windsor Castle to keep them safe from Nazis, it has been revealed.

King George VI order the secret operation to hide the jewels in an operation was so closely guarded that the Queen only learned exactly what happened to the jewels during the filming of a BBC documentary, The Times reported.

It was previously speculated that the jewels were taken from the Tower of London to Windsor during World War II or that they were kept in a vault in Canada. Others speculated that the jewels were taken to a secret tunnel at a Devon prison or a cave in Wales.

Royal commentator Alastair Bruce discovered the true story of the jewels' whereabouts during the war by reading confidential correspondence in the Royal Collection.

Bruce, who spoke to the Queen for a BBC documentary about the Coronation, discovered the jewels had been buried in a Bath Oliver biscuit tin with the grass left to regrow to conceal their hiding place.

The "electric set of letters," as they were described by Bruce, were from royal librarian Sir Owen Morshead to Queen Mary, the mother of George VI.

The letters reveal how deep a hole was dug beneath a sally port, one of the secure entries to Windsor Castle, and two chambers constructed with steel doors. The excavations were covered at night, the letters said.

"They dug out this fresh, very virgin white chalk and they had to hide it with tarpaulins so when the aircraft flew over at night no clue was given to the German Luftwaffe that anything was going on," Bruce said.

The Crown Jewels were locked inside, though access was available through a trapdoor that continues to exist.

Sir Owen described in his letter how the most precious jewels were removed from the Imperial State Crown and kept separately in case of emergency. The Black Prince's Ruby and St Edward's Sapphire were removed from their clasps and stored in the biscuit tin by Sir Owen.

The Queen was 14 at the time. Bruce told the Queen what had happened and she "had no knowledge of it," he said.

He added: "There had been in a book in our research that the Queen had been shown them during the war when they appeared in Windsor."

The book Operation Big: The Race to Stop Hitler's A-Bomb in 2016 said the jewels were known to be hidden at Windsor by the time in 1940 that the government was attempting to hide stocks of heavy water. However, it was unknown where the gems were buried or that they were in a biscuit tin.