A police officer guarding Prince George and his mother the Duchess of Cambridge left a dossier containing security information behind on a British Airways jet.
Cabin crew found the documents after the Royal party had left the flight and handed them to officials.
The blunder took place when the Duchess flew to the Caribbean at the end of January for the prince’s first holiday abroad.
They were accompanied by Kate’s brother James and sister Pippa and two Metropolitan Police Royalty Protection officers.
The group took a scheduled flight from Gatwick to St Lucia where they switched to a private plane for a trip to the luxury island of Mustique.
The lost file was discovered when they left the Boeing 777 to transfer to the smaller plane.
Scotland Yard today confirmed that an officer had left some documents on a plane.
The files - which police said were not classified documents - were returned to them within hours by airline staff.
A spokesman added: “The circumstances of the loss are being looked into.”
