People with O blood-type have been urged to make donations in the UK following a ransomware attack affecting data at major London hospitals.

Affected hospitals cannot currently match patients' blood at the same frequency as usual, but O blood-type is safe to use for all patients.

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

Memos to NHS staff at King's College Hospital, Guy's and St Thomas' (including the Royal Brompton and the Evelina London Children's Hospital) and primary care services in London said a critical incident had been declared.

Now NHS Blood and Transplant is calling for O-Positive and O-Negative blood donors to book appointments across the country to boost stocks.

Blood has a shelf life of 35 days, so stocks need to be continually replenished, the NHS added.

O-negative is the type that can be given to anyone, known as the universal blood type, and is used in emergencies or when a patient's blood type is unknown.

Just 8 per cent of the population have type O Negative but it makes up for around 15 per cent of hospital orders.

O positive is the most common blood type, 35 per cent of donors have it, and it can be given to anybody with any positive blood type. This means three in every four people, or 76 per cent of the population, can benefit from an O-positive donation.

Professor Stephen Powis, medical director for NHS England, said: "NHS staff are continuing to go above and beyond to minimise the significant disruption to patients following the ransomware cyber-attack on Synnovis earlier this week.

"But unfortunately, 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".