Hemel Hempstead town centre tricksters are believed to be conning people into parting with change in exchange for fake £20 notes.
Police are warning shoppers and businesses to be wary of people attempting to use and pass on fake currency in the town, after five incidents were reported in the past few days.
In each case, individuals have asked for change for £20, and those who have agreed have been given a counterfeit note in return.
One possible offender is described as a male aged between 20 and 30, of slim build with dark hair, and possibly sideburns.
PC Johanna Everett, investigating, said: “It is likely that there is more than one person involved and that not all of these incidents have been reported to us. I would urge anyone who believes they may have been approached by anyone trying to defraud them of money or believes they are in possession of fake currency to contact me.”
Call 101 with information.