Shopkeepers are being alerted to a scam that's seen around 50 shops lose up to €200,000 in the last week.
Some individual outlets have lost up to €14,000 in the scam, which involves callers using various accents ringing shops purporting to be an engineer from their card payment processer.
The Irish Payment Services Organisation (IPSO) said the caller advises shop staff to put through some refunds to what he assures them is a dummy card.
The scam is being investigated by IPSO and the Garda Bureau of Fraud investigation.
Shops already affected include petrol stations, jewellers, restaurants, chemists and florists.
Head of IPSO card services Una Dillon advised retailers to take down the card number given by the scammer, and to pass the number on to IPSO so that the card can be blocked.
The first reports of the scam came from Cork and Waterford, but IPSO has since received calls from all over the country from shopkeepers who fell foul of it.