CityWatch Lisburn has issued a warning about fake £20 notes circulating in the County Down area.
Local police have also issued a warning over the notes.
The concerns arose when a member of the public sold an iPhone for cash then later discovered that all the money was made up of fake Ulsterbank and Bank of Ireland notes.
Police said on two other occasions a fake £20 was used to get a cheap fast food meal which resulted in the individuals being given usable change.
Two arrests have been made for possession of fake currency and police will continue to investigate reports.