Blood stocks drop to 'unprecedentedly low levels' in England

At the same time, the number of people regularly giving blood tends to reduce in the summer, because more donors travel abroad on holiday.

Hot weather can also see an increase in people unable to donate because of a lack of hydration, or lower iron levels in their blood.

NHS Blood and Transplant is asking both O negative and O positive donors to urgently book an appointment online or on the telephone.

By 6pm on Thursday evening, the booking website, external was reporting high levels of demand, with potential donors being held in a virtual queue of up to 20 minutes before they could access the system.

The organisation also issued an amber alert for only the second time in its history.

That triggers contingency plans asking hospitals to restrict the use of O-type blood to essential cases, and allowing services to move staff to laboratories to carry out checks more quickly.

It says hospitals will still be able to perform urgent, emergency or trauma surgery, cancer surgery, transplant surgery and blood transfusions to treat people with long-term conditions.