Businesses in Mexborough, Denaby and Conisbrough are being warned to look out for fake £20 notes which have been circulating in shops and pubs.
Det Insp Paul Dickinson, of Doncaster CID, warned traders that false £20 notes are available on the black market at around one-third of their face value.
He said: “There is an increased incidence of false notes, particularly £20s being passed in Doncaster at the moment.
“I’d like to warn traders to be vigilant and, of course, report any they come across.”
A cache of £20 notes was found when police stopped a van in Doncaster last Friday.