extremely serious and unprecedented

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 gained control of the software that regulates the temperatures of the refrigeration units, forcing some to warm to 50C (120F) overnight, according to one supermarket chain affected.

The chief executive of Coopérative U said several of its supermarkets had been hit by the The cyber attack targeted software provided by Danish engineering firm Danfoss, which allows retailers to manage their cold units remotely. Normally, supermarket freezers go through a short period of defrosting overnight, in which temperatures rise slightly.

However, the hackers ordered the units to heat up and stay hot, although Danfoss did not confirm any reports about just how warm the fridges and freezers could become in that process. The affected cold units were older models whose security software could not be updated to ward off a risk they had identified.

Carrefour said fridges in 80 of its 6,000 stores were hacked, while Monoprix said 5% of its 400-plus stores had been hit, with food removed from sale if its condition could not be guaranteed. In a statement to the BBC, Mousquetaires Group - which owns Intermarché and Netto - said some fresh and frozen food display units remained out of service on Monday afternoon.

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. As a result of the attack, managers arriving at branches of several supermarket chains on Monday morning were hit by a wave of hot air from the freezers.

French prosecutors have opened a specific cyber-crime investigation into the incident, which the Paris prosecutor's office called a