Britain's Queen Elizabeth has advised wearers of crowns not to look down in case they injure their necks, in a rare interview for a BBC documentary on her coronation in 1953.

The Queen, 91, told the broadcaster her diamond-encrusted Imperial State Crown, which weighs 1.3 kilograms, was "very unwieldy".

The crown was made for the coronation of her father, King George VI, in 1937 and worn by the Queen for her own coronation and at state openings of parliament.

"Fortunately, my father and I have about the same sort of shaped head," she said in a clip from the documentary.

"But once you put it on, it stays," she said of the famous crown.

"And you can't look down to read the speech, you have to take the speech up.

"Because if you did (look down) your neck would break and (the crown) would fall off," the queen said, smiling as she tilted her head downwards.

"So there are some disadvantages to crowns but otherwise they're quite important things."

The Imperial State Crown features nearly 3000 diamonds, plus sapphires, emeralds and hundreds of pearls.

The Queen said she particularly likes the Black Prince's Ruby in the crown, which was reputedly worn by King Henry V in 1415 at the Battle of Agincourt.

She also said the golden ceremonial carriage used for her coronation was "horrible".

"It's not meant for travelling in at all," the Queen said. "It's only sprung on leather."