Dr Gail Miflin, chief medical officer, NHS Blood and Transplant, said: “Patient safety is our absolute priority.

“When hospitals do not know a patient’s blood type or cannot match their blood, it is safe to use O-type blood.

“To support London hospitals to carry out more surgeries and to provide the best care we can for all patients, we need more O negative and O positive donors than usual.”

Professor Stephen Powis, medical director for NHS England, said staff are going “above and beyond to minimise the significant disruption to patients” caused by the IT attack.

“We know that a number of operations and appointments have been postponed or diverted to other neighbouring hospitals not impacted by the cyber-attack, as we prioritise pathology services for the most clinically urgent cases,” he added.