The Queen's home films are to be broadcast to the nation for the first time, as she gives permission for rare private moments of her as a smitten young Princess to be seen.
The footage, unearthed within more than 400 reels of film watched by programme-makers, shows the Queen as a young woman, at the heart of a happy family before the weight of her public responsibility took hold.
It includes scenes of her gazing at an engagement ring just given to her by Prince Philip, before news of their betrothal was shared with the waiting world.
The films, described as "unseen private home movies", will be broadcast for the first time later this month, as part of the BBC's Platinum Jubilee celebrations.
Elizabeth: The Unseen Queen will "offer audiences the chance to witness rare private moments from the monarch's life", the corporation said, "telling the real story of her life as a Princess – through her own eyes and in her own words from across her reign".
Films will be narrated by the Queen
It will be narrated largely by the Queen herself, using clips and newsreel audio from her speeches to explain what viewers are seeing.
The 75-minute programme includes footage of the baby Princess Elizabeth being pushed in a pram by her mother, through to her Coronation in 1953 at the age of 27.
All the videos are believed to have been filmed by the Royal Family, firstly by the Queen's doting parents and later Prince Philip.
They will show scenes of her first tour abroad, to South Africa when she was 20, and footage of her with her Uncle George, the Duke of Kent who died in 1942.
Remarkable scenes will show the smiling Princess at Balmoral in 1946, admiring the engagement ring Prince Philip had designed for her using diamonds from a tiara given to him by his mother, Princess Alice.
The images are all-the-more poignant for being shot before news of the glamorous young couple's engagement spread, as they shared their private joy with King George VI and Queen Elizabeth during Philip's first extended stay with them in Scotland.
Later, Princess Elizabeth will be seen as a young mother, spending time with the King and Queen in their new roles as "fond grandparents" to Prince Charles and Princess Anne.
Other footage from Balmoral, taken in 1951, captures the King's final visit there before his death, while home film reels show the Queen's grandmother George V - known to her as "Grandpa England" - sailing off the Isle of Wight in 1931.
Footage takes public 'inside Her Majesty's life as never before'
The BBC said the Queen had granted "unprecedented access" to the films, "taking us inside Her Majesty's life as never before through home movies shot by her, her parents, Prince Philip, and others".
Programme-makers promise to shed light on her warm relationship with her parents, as well as her fairytale romance with the Duke of Edinburgh.
In viewing more than 400 reels of film, producers discovered lost footage from behind-the-scenes at state events, believed to have been privately commissioned by the Royal Family and given to the Queen.
They also listened to more than three hundred of the Queen's speeches across eight decades of public life.
In common with all royal videos since the 1920s, the homemade recordings had been stored carefully in the Royal Collection vaults of the British Film Institute, with no promise of being aired.
The Platinum Jubilee presented a worthy moment to air them to the public.
"The result is a visually rich and immersive film of unparalleled historical record," said the BBC.
Claire Popplewell, creative director for BBC Studios events productions, said: "As programme-makers who have previously worked closely with the Royal Household on ceremonial and celebratory broadcast events and programmes, the production team were under no illusion quite how special having access to this very personal archive was.
"Being able to draw upon the self-recorded history of a young Princess Elizabeth and her wider family - and allowing The Queen to tell us her own story - is the very heart of this film."
Simon Young, the BBC's commissioning editor for history, added: "We are honoured that the Queen has entrusted the BBC with such unprecedented access to her personal film collection.
"This documentary is an extraordinary glimpse into a deeply personal side of the Royal Family that is rarely seen, and it's wonderful to be able to share it with the nation as we mark her Platinum Jubilee."