Owners of chippies and other takeaways are being warned by police after a "strange" new crime trend has emerged.
Police officers say they are seeing more and more cases of used cooking oil being stolen.
Often this stolen oil is is then converted into unregulated biodiesel for the black market.
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In November last year, police in North Wales stopped and arrested two men from London on the M1 in connection with the theft of cooking oil in Betws-y-Coed, Conwy.
The North Wales Police (NWP) West Conwy Coastal said the crime was not new but it had recently developed into something of a “trend”, North Wales Live reports.
On Facebook, the team said: “We have been made aware of a crime trend involving the theft of used cooking oil recently.
“Although this would seem a strange thing to steal, used cooking oil has value for things like biofuels and other purposes.
“We would ask all food outlets in the area to be alert for this type of crime.”
Used vegetable oil from commercial operators cannot be poured down the drains and must be collected by a waste carrier.
Often it is kept in large storage tanks at the back of premises which are then targeted by thieves.
Producing black market biodiesel from stolen edible oil can cost just a fraction of biodiesel made from legitimate oil.
However "chip shop fuel" can damage modern diesel engines and can clog up vital components such as fuel injectors and emission control devices.
Police forces across the UK have reported recent rises in thefts involving by organised gangs.
It is thought that stolen oil is being taken to the south of England for processing.
As a result, food outlets are being told to keep backyards securely locked and to consider installing CCTV and security lighting.
The NWP West Conwy Coastal team warned businesses to watch out for bogus collectors.
It said: “Please secure all used cooking oil.
“If you have a contract to have it collected, make sure you check to confirm that it is that legitimate company that you are dealing with and not a criminal gang pretending to be from a real waste company.”
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