London's police force said on Saturday it had suspended an officer tasked with guarding British Foreign Minister Dominic Raab, following a newspaper report that the officer had left a loaded gun on a plane.
The incident is the second time this year that a British police officer has been suspended in similar circumstances. One of former Prime Minister David Cameron's bodyguards mislaid his firearm and Cameron's passport in a plane toilet in February.
Britain's Sun newspaper said a cleaner at London's Heathrow Airport found a semi-automatic Glock 19 pistol on a United Airlines plane which had just landed after an overnight flight from Washington.
Raab had been travelling back to Britain on the flight after meeting U.S. Secretary of State Mike Pompeo.
"We are aware of the incident on a flight into the UK on Friday 18 September and we are taking this matter extremely seriously," London's Metropolitan Police said in a statement.
"The officer involved has since been removed from operational duties whilst an internal investigation into the circumstances is taking place," it said.
The Sun said the officer had been sorting out passports when he took off his holster with the pistol in it, and left it on the seat of the plane.
A police spokesman said he could not comment on the details of the Sun's report.