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In June 2000 a royal protection officer accidentally fired two shots on the royal train while the 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.
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.
The Met was keen to stress that officers were some distance away from the Duke and Duchess of Cambridge's home when the royal protection officer accidentally fired a gun and that no one else was at risk.
Prince William or Flight Lieutenant Wales as he is known in his capacity as a RAF search and rescue helicopter pilot, captains Sea King helicopters from his unit's base at RAF Valley in north Wales.
Police would not comment on the type of gun, but royal protection officers routinely use 9mm Glock 17 pistols.
Latest ITV News reports
An investigation has begun after a policeman believed to be guarding the home of The Duke and Duchess of Cambridge accidentally fired a gun