Operations have been canceled at several of London’s largest hospitals, and a critical incident emergency status declared, following a ransomware attack on a third-party provider leaving healthcare professionals without access to pathology services.

The attack, which was detected on Monday, impacted a company called Synnovis that provides pathology services, such as blood tests for transfusions, to a number of healthcare organizations, according to reports and internal emails published on social media.

“I can confirm that our pathology partner Synnovis experienced a major IT incident earlier today, which is ongoing and means that we are not currently connected to the Synnovis IT servers,” wrote Ian Ebbs, the chief executive at Guy’s and St Thomas’ NHS Foundation Trust, a hospital network.

Royal Brompton and Harefield hospitals, the largest specialist heart and lung centers in the United Kingdom, are also believed to be affected. The incident is also affecting King’s College Hospital NHS Foundation Trust “and primary care across south east London,” wrote Abbs, “having a major impact on the delivery of our services, with blood transfusions being particularly affected.”