The police protection officer travelling with the foreign secretary has been taken off his operational duties while an investigation into the incident takes place.
Dominic Raab had been in Washington DC to talk to US politicians about Brexit.
A Metropolitan Police spokesperson said: “We are aware of the incident on a flight into the UK on Friday September 18 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.”
The gun was reportedly found by a cleaner.
Earlier this year, a bodyguard for former prime minister David Cameron was investigated for leaving his gun in a plane toilet.
Cameron is entitled to continued security as a former prime minister and in that incident, the gun was discovered by a passenger on a British Airways flight from New York to London.
This article originally appeared on HuffPost UK and has been updated.