The ransomware attack on a key U.K. National Health Service IT vendor has forced two London hospitals to reschedule around 1,500 medical appointments including critical cancer treatments and organ transplant surgeries.

The June 3 attack is disrupting operations at NHS King’s College and Guy’s and St. Thomas’ in London. Attackers compromised servers of Synovia, the main pathology services vendor to the two hospitals (see: UK Vendor’s Attack Disrupts Care at London NHS Hospitals).

The attack severely disrupted hospital services including blood testing at King’s College Hospital, forcing the facility to temporarily prioritize patients with urgent needs. NHS Blood and Transplant called on O positive and O negative blood donors to book appointments with the 25 NHS Blood Donor Centers to boost blood supplies in the wake of the hack.