King’s College Hospital is calling on staff with the blood type O to schedule donations, according to an internal memo sent to staff and reviewed by Bloomberg News. The hospital declined to comment. National Health Service, the publicly funded health care system in England, didn’t respond to a request for comment.

The letter followed a broader appeal issued by NHS England to the general public for blood donations and is only the latest fallout from a ransomware attack that has plagued several of the region’s medical centers since June 4. The hack against Synnovis, a provider that helps manage blood transfusions and testing for the hospitals, has forced some facilities to delay medical operations, postpone blood samples and resort to handwritten records.

“NHS Blood and Transplant is calling for O blood type donors (both positive and negative) to book appointments in order to support the response to the ransomware cyber attack incident,” Julie Lowe, deputy chief executive of King’s College Hospital NHS Foundation Trust, said in the internal memo. “Staff are also encouraged to donate blood as well.”

O type blood is particularly critical. O negative blood can be given to anyone regardless of their own blood type, and O positive is the most common type. The attack has meant hospitals can’t match patients’ blood at the same frequency as usual, NHS Blood and Transplant said Monday in a statement.

The memo also warns that disruptions tied to the hack were expected to continue for the rest of the week, with the trust redirecting “a small number of patients” to other hospitals. Some planned operations and outpatient appointments will likely be canceled, Lowe said. “We still do not know when Synnovis’ systems are likely to be restored, so we should expect to be dealing with this incident for the rest of this week, and beyond,” she added.

The attack on Synnovis, a partnership between Synlab UK & Ireland, Guy’s and St Thomas’ NHS Foundation Trust and King’s College Hospital NHS Foundation Trust, has primarily affected patients of Guy’s and St Thomas’ Hospital, King’s College Hospital and primary care in southeast London. The lab services provider has said it is working with NHS England to minimize the impact on patients. 

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The fallout of the attack has rippled across to primary care providers whose practices rely on the affected hospitals for laboratory services. 

Azeem Majeed, a physician in south London, said local practices and community services have historically ordered up 10,000 blood samples a day for testing. Providers are now restricted to requesting around 400 a day, he said. 

Routine blood tests for patients with conditions including high blood pressure, diabetes and kidney diseases are facing delays, he said, while noting that patients with urgent issues are being prioritized.