The officer had been travelling with Mr Raab on a visit to the US when he left his gun on a plane at London’s Heathrow Airport on Friday.
The gun was reportedly found by a cleaner.
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.”
In a similar incident in February this year, former prime minister David Cameron’s bodyguard left his gun in a plane toilet.
Passports belonging to Mr Cameron and the officer were also found alongside the weapon, according to newspaper reports at the time.
The gun, believed to be a 9mm Glock 17 pistol, was found by a passenger, who handed it to flight attendants on a transatlantic flight.
Mr Raab visited Washington this week to speak with senior figures including US Secretary of State Mike Pompeo on issues of Brexit and Northern Ireland.
