For a decade, Queen Elizabeth II lived without knowing that her own art advisor, Anthony Blunt, a trusted man in charge of overseeing the invaluable Royal Collection, was living a double life as a Soviet agent.

Documents newly declassified by MI5, Britain's internal intelligence agency, show the late Queen Elizabeth II was not informed for almost 10 years that a member of her staff had spied for the then-Soviet Union.

The identity of a Russian spy among Queen Elizabeth II's courtiers was kept from the monarch for almost a decade, but she took the news with her typically calm demeanour