A Royal Protection officer has been arrested following the discovery of ammunition in personal lockers in the grounds of Buckingham Palace.
Scotland Yard said the ammunition is believed to be from the Metropolitan Police's own supplies.
The arrested officer is being questioned on suspicion of misconduct in public office and unlawful possession of ammunition.
He is in custody at a London police station and will be suspended from duty.
The force said the investigation, by the Directorate of Professional Standards, began a few weeks ago after officers from Royalty Protection reported ammunition found in their personal lockers or belongings.
The officer arrested is from the unit responsible for protecting the Royal Household (SO14) and normally works in a protection role, although he is not assigned to guard any individual member of the Royal Family.
Investigators have been searching the officer's home address and the dedicated police building in the grounds of the palace where the ammunition was found.
Police said the Royal Household is aware of the ongoing investigation and has been briefed on today's development.
The force added that the arrest has not affected the established security procedures surrounding the daily work of the officers providing personal protection to the Royal Family, who are some of the "most highly trained and professional officers in the Met".
The Independent Police Complaints Commission has also been informed.
The arrest follows the announcement last week that the Met was reorganising its Protection Command, which is also responsible for guarding key political figures.
Assistant Commissioner Mark Rowley admitted then that a number of scandals are being investigated - including claims that police at Buckingham Palace had stolen confiscated weapons.
He also said force chiefs want to disrupt "one or two pockets" where officers have become "overly comfortable and familiar" by moving staff who have been in their jobs for several years.