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.
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A number of families were on board the service from London Euston to Stoke. One family left the train early to escape the abuse.
She's called Amy, and was visiting a hospital.