Dominic Raab's bodyguard has been "removed from operational duties" after allegedly leaving his gun on a plane at Heathrow.
A cleaner is reported to have found the loaded Glock 19 pistol in its holster on a seat on the United Airlines flight after it landed in London on Friday (UK time).
The foreign secretary had been in Washington DC to speak to US politicians about Brexit.
Scotland Yard said an internal investigation was taking place.
The cleaner raised the alarm after finding the weapon, The Sun reported, and police boarded the plane - before realising the gun belonged to one of Raab's close protection officers.
A Metropolitan Police spokesman said: "We are aware of the incident on a flight into the UK on Friday 18 September and we are taking this matter extremely seriously.
"The officer involved has since been removed from operational duties whilst an internal investigation into the circumstances is taking place."
'Lapse in concentration'
The Sun reported that the officer took off his holster and put it on his seat before escorting the foreign secretary off the flight - leaving it behind.
Former Met chief superintendent Nick Aldworth told the BBC there were strict rules about the handling of firearms, but that human error does happen.
"This sort of thing is not common, but it does happen," he told the Today programme. "It happens because human beings are in the system and human beings operate under an enormous amount of stress sometimes, particularly when they are managing armed operations or in the middle of an armed operation.
"This appears to be an unfortunate lapse in concentration for which the officer will pay a price in their career, in as much as they have been removed from operational duties, which is something they probably love doing, and sadly for them, will no longer be able to do."
He added that the Met takes such matters "incredibly seriously".
In February, a bodyguard for former prime minister David Cameron was investigated for reportedly leaving his gun in a plane toilet.
In that incident, the gun was found by a passenger on a British Airways flight from New York to London. Cameron is entitled to continued security provided by the Metropolitan Police as a former prime minister.