Four police officers from the Royalty Protection unit of the Metropolitan Police are being investigated over an allegation of weapons property being mishandled.
One sergeant has been placed on restricted duties, a spokesman for Scotland Yard said, after an investigation was launched in June.
A statement from police said: "Officers from the Directorate or Professional Standards are investigating an allegation of mishandling of property from the weapons disposal bin by SO14 officers.
"Four officers, three sergeants and one PC, are the subject of the Directorate of Professional Standards investigations."
None of the officers being investigated was arrested or suspended, a Met spokesman said.
SO14 is the branch of the Met responsible for safeguarding members of the royal family and their residences including Buckingham Palace.
Weapons disposal bins are located at all royal residences open to the public and visitors are checked on arrival.
The bin in relation to this investigation was reportedly located at Buckingham Palace, although a Met spokesman said he could not confirm this.
Small knives and scissors are among items which must be checked in on a visitor's arrival at the palace and can be reclaimed on departure, the Royal Collection Trust website states.