London's Metropolitan Police said on Saturday it had suspended an officer charged with protecting Foreign Secretary Dominic Raab, after a newspaper published a report saying he had left a loaded pistol on a plane.
The incident is the second of its kind this year in which a British policeman was suspended from work under similar circumstances.
One of the bodyguards of former Prime Minister David Cameron lost his weapon and Cameron's passport, after forgetting them in the bathroom on a plane.
The British Sun newspaper reported that a cleaning worker at London's Heathrow Airport found a semi-automatic "Glock 19" pistol on a United Airlines plane that had arrived after a flight it made overnight from Washington.
Raab was returning home on that trip after meeting his US counterpart, Mike Pompeo.
"We are aware of this incident on a flight coming to Britain on Friday 18 September, and we are taking the matter very seriously," the Metropolitan Police said in a statement.
She added, "We have relieved the officer in question of his duties since then, while an internal investigation into the circumstances is underway."