Archive footage of the Queen and Queen Mother apparently raising a Nazi salute in 1933 may have been inadvertently given to documentary makers filming a royal family tribute.
Buckingham Palace is understood to be investigating how the material ended up in the hands of the Sun.
It is considering all avenues, including theft and possible breaches of copyright. It may have been handed over by mistake.
The tabloid published a 17-second excerpt, seemingly showing the future Queen, aged just 7, and her mother performing a Nazi salute.
The Queen's uncle - who became Edward VIII, later abdicated and was accused of being a Nazi sympathiser - is also featured in the footage performing the salute.