Hundreds of French supermarkets have been hit by a cyber attack that caused their fridges and freezers to heat up, spoiling fresh and frozen food estimated to be worth millions of euros.

Hackers reportedly gained control of the software that regulates the temperatures of the refrigeration units, forcing some to warm to 50C (120F) overnight.

The chief executive of Coopérative U said several of its supermarkets had been hit by the “extremely serious and unprecedented” attack.

More than 100 of the chain’s supermarkets were targeted, while at least 300 stores from Intermarché and its Netto brand were affected, according to the AFP news agency.

Intermarché said its fridges had heated up to 50C for six hours overnight into Monday, and the retailer had thrown away all the affected food to avoid health risks, the Reuters news agency reported.

Some supermarket staff reportedly said they were hit by waves of hot air when they opened freezer units on Monday morning.

French broadcaster BFMTV published an image showing a freezer in an ice cream aisle displaying a temperature of 26.5C.

Two supermarkets in the Alsace region reported losses of €270,000 (£228,000) between them, the broadcaster added, meaning the nationwide total could run into the millions.

The Paris prosecutor’s office told AFP it had opened an investigation into the incident, which it called a “large-scale cyber-attack affecting the refrigerators of several major food retail chains across France”.

Eoropean Food Commissioner Christophe Hansen condemned the hack in a post on X and added: “Food security requires more attention at all levels. Because the weaponisation of food has become a reality.”

The full scale of the attack is so far unclear.

Reports in specialist retail and local media said stores from other food giants, including Carrefour, E.Leclerc and Monoprix, were also attacked. BBC News has approached the supermarket chains for comment.

In a post on X, Coopérative chief executive Dominique Schelcher said staff were “fully mobilised, in co-ordination with refrigeration specialists, to restore the situation as quickly as possible”.

He called for an investigation into the hack and said targeting equipment essential to the French food supply was “unacceptable”.