Around 40 Delhaize supermarkets in Belgium and Luxembourg were affected, while a smaller number of shops were hit in Sweden, Ireland, the Netherlands, Estonia and Finland.

Sabcobel, a Belgian company that manufactures some of the affected refrigerators, said the units' evaporators were switched to defrost mode, causing them to warm up, in what is believed to be the same method used in last month's incident that hit hundreds of French supermarkets.

Millions of euros worth of food had to be thrown away in October, local media reported at the time.

Stephan Moelans, the CEO of Sabcobel, which operates Delhaize's systems, on Monday told the Belga news agency that the incident was a cyber-attack and that the company had been "working all night and morning to disconnect the affected units from the network".

He said since the French attack in early October, the company had been working to "protect our customers fridges against this vulnerability" but that the units needed physical modifications, not just software updates.

Most supermarkets across Europe are linked to the internet, and maintenance and troubleshooting can be done remotely.

"The simplest solution is to manage the fridges over a secure internet connection. Most of our customers switched to that after the attack in France," Moelans added.

The temperature of freezers in several shops across Europe reportedly rose to as high as 50C (122F).