A missing sister, a plane that vanished and a secret lover: Teresa Drake's disappearance sparked global headlines. Now her family reveal she may have been an MI6 spy killed by the US… and have been warned not to 'ask questions'

On the evening of August 11, 1979, British secretary Teresa Drake boarded a private plane from Athens to Jeddah and was never seen again. Just months before, the 22-year-old had left a post in the Army.