A policeman believed to have been guarding the home of The Duke and Duchess of Cambridge accidentally fired a gun while sitting in an unmarked car.
Nobody was injured when the officer - who was in the vehicle with another policeman - fired the shot while on duty in north Wales last week.
Scotland Yard confirmed there would be an investigation into the incident.
A Clarence House spokesman said he could not confirm where the incident had taken place.
The officer concerned has been removed from firearms duty pending the outcome of the inquiry and the Met's Directorate of Professional Standards has been informed.
Both officers are attached to Specialist Operations, the Met said.
In June 2000 a royal protection officer accidentally fired two shots on the royal train while Queen and Duke of Edinburgh were on board.
The incident occurred in the early hours of the morning when the royals were asleep in another carriage - no one was injured in the accident.
In November 2008 one of Tony Blair's bodyguards accidentally fired his gun as the former Prime Minister prepared to board a flight at Ben Gurion airport in Israel. No one was hurt in the incident, with the bullet hitting the ground.
Prince William captains Sea King helicopters from his unit's base at RAF Valley in north Wales.