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The permanent staining liquid, with its unique code, is virtually impossible to remove, say police, and gives them conclusive proof of ownership

The City of London police are using traceable permanent UV dye, SmartWater, in sting operations to help officers catch bike thieves in the Square Mile.

Last year on average 38 bikes were stolen per month in the Square Mile and to help curb the growth in theft police are placing 'trap bikes' with SmartWater around the City.

The permanent liquid, which police say is almost impossible to remove, and is only visible under UV light, comes with a unique 'code' which is traceable to a specific location.

Temporary Superintendent Helen Isaac from the City of London Police said, “The use of ‘trap bikes’ has been very successful in other force areas in helping the police catch and prosecute offenders. We hope this method will help decrease the number of pedal cycles stolen in the City of London.

“The bikes will be marked with a transferrable form of SmartWater which will mark the skin and clothing of the thief, directly linking them to the stolen bike.

“Signs warning of the use of SmartWater and ‘trap bikes’ will be placed in hotspot areas giving a clear message to thieves to stay clear, otherwise they will end up being arrested.”

Forensic tests on stolen goods retrieved allow police to conclusively prove ownership of marked goods, they say.

SmartWater on a mobile phone under UV light

The force also offers free bike marking with BikeRegister.com - information on marking events is available via Twitter accounts @citypolice or @citycyclecop.

In March the Mayor of London, Boris Johnson, announced the world's largest rollout of SmartWater to reduce burglary rates across the capital. In one trial across five boroughs use of the liquid reduced burglaries by 49%, with some areas seeing reductions of 70%.