Co-op has warned customers of ongoing product shortages across its store network as the retailer continues to battle a “sustained” cyber attack that has forced it to shut down some of its delivery systems.

The retailer said the attack on its systems, which began last week, has caused serious disruption to store availability, particularly in fresh categories such as fruit, vegetables, meat and dairy.

According to reports, hacking groups Dragonforce and Scattered Spider—allegedly composed of British and US teenagers—are believed to be behind the Co-op and M&S breaches. Co-op’s own cybersecurity lead was reportedly contacted directly by hackers through an internal chat platform.