Video report by ITV News Royal Editor Chris Ship
When it was first broadcast in 1969 – initially on the BBC and then on ITV - three-quarters of UK population watched it.
The documentary was a significant enough punctuation in Queen Elizabeth’s long reign that Netflix’s The Crown featured it in one of its episodes.
But the famous 90-minute film called Royal Family has never been repeated at the Queen’s request – that was until it mysteriously found its way onto YouTube this month.
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The entire film was uploaded and viewed more than 10,000 times before it was taken down yesterday after intervention from the BBC following what YouTube called a "copyright claim".
The copyright of the film actually rests with the Palace but its removal from YouTube yesterday was too late to prevent it from being copied.
It means extracts from the film are now likely to find their way onto the internet multiple times destroying the secrecy which has surrounded the documentary for many decades.
The BBC declined to comment when we approached them and Buckingham Palace is reluctant to comment directly.
But a royal source did say: "From time to time, things pop up on the internet that should not be there. We will assume it’s going to be taken down."
Despite the five decades which have passed since its broadcast it retains its mystique to this day which is why Buckingham Palace is keen it was not available on YouTube for the whole world to see.
In the family itself, the Duke of Edinburgh was the most enthusiastic member about the project when the palace was approached about it at the end of the swinging sixties.
Prince Philip was worried that after the liberating years of the 1960s, the Royal Family was in danger of appearing unrelatable, out-of-touch, stuffy and elitist.
He was the one who had pushed hard to allow the cameras the film the 1953 coronation and was constantly pushing to modernise the institution of which his wife was the Monarch.
So he persuaded the Queen to give the project the green light.
The cameras were allowed to film the family at the breakfast table talking about King George VI swearing whilst cutting down rhododendron bushes at Sandringham.
The Queen was filmed buying a young Prince Edward an ice cream in the local post office near Balmoral.
And the cameras rolled as Prince Philip cooked sausages for the family at one of his famous outdoor barbecues.
After the recent debate about the blurring of fact and fiction in The Crown, you might have thought palace aides would want something on the internet which is undoubtedly fact.
But royal sources say that documentaries in which family members have agreed to take part are rarely shown on multiple repeats.
They believe that scarcity is what makes them interesting.
And on the matter of the 1969 film, scarcity is something they are still seeking even in this internet age.