One possibility is that the footage of the Queen and Queen Mother apparently raising a Nazi salute in 1933 could have been inadvertently handed to film-makers for a tribute documentary, as part of batch of Royal home movies.
Buckingham Palace is understood to have begun an investigation into how the material ended up in the hands of The Sun newspaper. It is considering all avenues, including whether the film was stolen, while also looking at possible breaches of copyright. It may even have been handed over by mistake.
The tabloid on Saturday published a 17?second excerpt from the footage, seemingly showing the future Queen, aged just seven, and her mother performing the Nazi salute. The Queen's uncle, the then Edward, Prince of Wales, who would later be accused of being a Nazi sympathiser, is also featured in the footage performing the salute.
It is thought that the future King George VI, the Queen's father, was behind the camera. The footage is understood to be part of a vast collection of photographs, correspondence and home movies that make up the Royal archive, housed at Windsor Castle.
The Sun has refused to disclose the source but said "the original film remains under lock and key". It added that "several copies of the clip were made several years ago" and that one copy had been handed to The Sun.
Senior sources told The Telegraph that it was inconceivable that anybody working in the Royal archive would have leaked such material because of their "loyalty" to the Royal family. Outside access to the archive is "rare" and "tightly controlled".
Film-makers make requests for access to home movie footage for documentaries including tribute programmes for such events as the Queen's golden and diamond jubilee celebrations among others.
"This probably happened when somebody was releasing some footage to a television company and somebody at the Royal archive didn't see it could be misinterpreted or else didn't check carefully for what was in it," said one source. The except obtained by The Sun is part of a longer home movie of the then Princess Elizabeth and her family playing in the grounds of Birkhall, on the Balmoral estate.
The Sun is understood to have been in possession of the footage for some weeks. Buckingham Palace was first made aware of its existence on Thursday evening, prompting an angry exchange of letters. The Sun chose to publish on Saturday, to the fury of Buckingham Palace, which accused the newspaper of exploiting the Royal family's private archive.
The Queen Mother's official biographer, William Shawcross, accused the newspaper of a "clear" breach of privacy.
Mr Shawcross said: "I think it is absolutely outrageous that The Sun should publish this. It adds nothing to our historical knowledge. It is children playing with their parents. Anybody can horse around in their back garden. It means absolutely nothing.
"When I wrote the official biography of the Queen Mother I spent six years in the Royal archive going through the Queen Mother's private letters and there is not a scintilla of evidence of Nazi sympathies in her letters.
"She wrote of the evil of Nazism. She and the king did more than anyone apart from Churchill to keep up morale during the war.
Mr Shawcross added: "There is no way anybody in the archives has leaked this or anything else. They are totally scrupulous, honourable people."
Dickie Arbiter, the Queen's former press secretary, said: "I would like to think it was released inadvertently as a bit of harmless 1933 footage without anybody really knowing what was on it. I think what they [Buckingham Palace] would probably like to know is where it came from and who gave it to The Sun."
The Sun's managing editor, Stig Abell, said the footage was obtained by the newspaper "in a legitimate fashion" and that its publication was "not a criticism of the Queen or the Queen Mum".
"It is a historical document that really sheds some insight into the behaviour of Edward VIII," he told BBC Radio 4's Today.
Asked about the complaint by the Palace that it had been "exploited", he said: "I understand that they don't like this coming out but I also feel, on a relatively purist basis, that the role of journalists and the media is to bring to light things that happened."
