The NHS aims to collect 1.45 million units of blood annually.

This requires up to 200,000 new blood donors each year to meet patient needs and maintain a diverse range of blood groups.

Mark Chambers, director of donor experience at NHS Blood and Transplant, said: “Demand for lifesaving blood doesn’t stop for Christmas.

“Many patients will need transfusions over the festive period as treatment for a blood condition or cancer, or due to surgery, childbirth, or an accident.”

The capital’s permanent donor centres, including Shepherd’s Bush, Stratford, Tooting, Twickenham and West End, have a combined 4,046 available slots over the next fortnight.

Mr Chambers also urged those who have not donated before to give blood for the first time.

“It only takes an hour to give blood, and each donation can help save or improve up to three lives,” he said.

The NHS urged donors to provide as much notice as possible for cancellations, so other donors could take up the slot.

It also said black heritage donors were urgently needed as they are more likely to have the blood type needed to treat the growing number of patients with sickle cell disease.