Pickpockets are preying on shoppers carrying canvas bags by slashing them with a knife to get access to valuables.
Crimefighters issued the alert in the run-up to the Easter break as hundreds of people prepare to make the most of the four-day holiday.
The method has been used by cunning crooks in Colchester in the past, and a failed attempt was reported to police by a woman last week.
Colchester crime reduction officer Colin Stiff said: "We do know about this method of operation and this has been known to happen in the past, albeit rarely.
"They tend to use a pocket knife or something like that to rip the bag so they can grab valuables quietly.
"It is not unknown for this to happen, and it is just another way they can get to valuables.
"The problem is when we try to reduce crime, we have to talk about crime and be careful not to give anybody ideas, but they know who their targets are, and to stop them people need to be made aware."
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