A bodyguard for David Cameron is being investigated after he reportedly left his gun in a toilet on a transatlantic jet.
A "terrified passenger" found the gun and gave it to staff on a British Airways flight from New York to London on Monday, according to the Daily Mail.
As a former prime minister, Mr Cameron is entitled to continued security provided by the Metropolitan Police.
The Met said the officer involved has been removed from operational duties.
Mr Cameron's team said it could not comment on security matters.
The gun, believed to be a 9mm Glock 17 pistol, is said to have been left by a close-protection officer from the Met's Specialist Protection unit, who took off his holster while in the toilet.
Mr Cameron's passport - and that of the officer - were found with the weapon, according to the Sun.
A Metropolitan Police spokesperson said: "We are aware of the incident on a flight into the UK on 3 February and the officer involved has since been removed from operational duties.
"We are taking this matter extremely seriously and an internal investigation is taking place."
British Airways said it had adhered to Civil Aviation Authority rules allowing UK police "to carry firearms on board in specific, controlled circumstances".
"Our crew dealt with the issue quickly before departure and the flight continued as normal," the airline added in a statement.
Mr Cameron was the UK prime minister for six years until July 2016, when he stepped down following the result of the EU referendum.
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