A CS spray canister dropped during an operation to catch a thief in Park South is still missing.
A police officer lost the spray in a confrontation in Cavendish Square.
Despite a thorough search of the area the canister wasn't found and police are concerned it could end up in the wrong hands.
Swindon police have been using CS spray since April 1998 and it is now standard issue for operational staff.
It contains three main ingredients, the active chemical CS, a liquid solvent and an inert gas to act as a propellant. Police are warning anyone who finds the spray to return it to police straight away or risk committing a serious offence.
Anyone with information on the whereabouts of the missing canister is urged to call police on Swindon 528111.
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