NHS Blood and Transplant is appealing for O blood-type donors to book appointments amid the cyber attack affecting major London hospitals.

The IT attack means that the affected hospitals cannot match patients' blood at the same frequency as usual.

Several London hospitals declared a critical incident, cancelled operations and tests, and were unable to carry out blood transfusions last week after the attack on the pathology firm Synnovis, which Qilin, a Russian group of cyber criminals, is understood to have been behind.

NHS Blood and Transplant is calling for O positive and O negative blood donors to book appointments in one of the 25 NHS Blood Donor Centres, external in England to boost stocks.

These blood types are used when a patient's blood type is unknown, and is carried in emergency response vehicles.

Dr Gail Miflin, chief medical officer at NHS Blood and Transplant, told the BBC: "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."