Police have warned people to be on the lookout for counterfeit notes after several reports of counterfeit banknotes in circulation in Londonderry, Limavady, Strabane and Castlederg over the past few weeks.
Counterfeit £10 and £20 banknotes have been used in some licensed premises and shops.
Earlier this week a bar in the Strand Road area of Derry reported counterfeit notes being passed on and in a follow-up operation, police officers recovered a substantial quantity of notes during a search of a vehicle.
Three men were arrested and later released pending further inquiries.
On Wednesday several hundred pounds worth of counterfeit notes were passed in a city centre shop.
The notes recovered by the police were £10 and £20 of Bank of England notes, Bank of Ireland notes and Northern Bank notes.
A PSNI spokeswoman said: “Police would always advise local business people, staff and members of the public to be vigilant and take a few minutes to check notes carefully before accepting them and always report any suspicious incidents to police.
“It is a criminal offence to try to pass or keep a banknote you either know or suspect to be counterfeit.
“If you inadvertently receive a note that you believe to be counterfeit, you should take it to the nearest police station.
“Anyone who has any information regarding the production or distribution of counterfeit banknotes, can contact police by calling 0845 600 8000 or alternatively contact the Crimestoppers charity anonymously on 0800 555 111.”
A ‘Know Your Banknotes’ leaflet can be obtained from branches of all main banks.