Speaking for the first time about her coronation 65 years ago, Queen Elizabeth II has revealed how uncomfortable she was riding in her golden carriage to the ceremony, and how wearing the Imperial State Crown risks "breaking your neck".

Key points: Queen Elizabeth II hid British crown jewels in biscuit tin during World War II

Queen Elizabeth II hid British crown jewels in biscuit tin during World War II She speaks candidly about her coronation for the first time in a BBC documentary

She speaks candidly about her coronation for the first time in a BBC documentary The Queen said the Imperial State Crown risked breaking her neck

In a very rare, personal account for a BBC documentary to be aired on Sunday (local time), Britain's longest-reigning monarch, who was crowned queen in 1953, spoke candidly about her coronation and some of the crown jewels that play a symbolic role in the ceremony.

"Horrible," she said of the ride in the 4-tonne carriage from Buckingham Palace to Westminster Abbey, where English monarchs have been crowned since 1066.

"It's only sprung on leather, not very comfortable."

Queen Elizabeth II inspects the diamond-encrusted Imperial State Crown. ( BBC )

Queen Elizabeth, now 91, was just 25 when she became queen on the death of her father George VI in 1952, with the coronation taking place the following year.

"It's the sort of, I suppose, the beginning of one's life really as a sovereign," she said.

"It is sort of a pageant of chivalry and old-fashioned way of doing things really.

"I've seen one coronation [her father's in 1937] and been the recipient in the other, which is pretty remarkable."

Giving her personal recollection, the Queen also revealed how she had struggled with her coronation dress, which was embroidered in silk with pearls, and gold and silver thread.

"I remember one moment when I was going against the pile of the carpet and I couldn't move at all," she said.

The Queen's coronation dress. ( ABC News: Kate McCure )

The documentary also features informal footage taken behind the scenes, including images of son and heir Prince Charles, then aged four, and his younger sister Anne playing underneath the queen's long robe.

"Not what they're meant to do," the Queen quipped.

Queen hid jewels in a biscuit tin

Queen Elizabeth II and Prince Philip, Duke of Edinburgh, after her coronation. ( Supplied: Library and Archives Canada )

Prince Charles has previously spoke of how his mother had practised wearing the 2.2-kilogram St Edward's Crown while he was being bathed.

The Queen wore two crowns for her coronation: the St Edward's Crown, which she has never worn since, and the diamond-encrusted Imperial State Crown, which she wears at formal occasions such as the opening of parliament when she delivers a speech outlining the government's legislative plans.

"You can't look down to read the speech, you have to take the speech up. Because if you did, your neck would break and it [the crown] would fall off," she said smiling.

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

The documentary has also revealed some of the most valuable gemstones of the British crown jewels were kept hidden underground in a biscuit tin during World War II.

It was known that the crown jewels had been taken to Windsor Castle, but during the BBC program it emerged they were kept in a hole dug under a secret exit from the castle.

The Queen has never given a formal interview during her long reign.

Reuters

You can watch The Coronation on the ABC on Sunday, February 4 at 7.40pm AEDT.