Blood supplies have fallen critically low, forcing the NHS department that manages donations to declare its first-ever amber alert status.
NHS Blood and Transplant (NHSBT) said overall blood stocks in the health service stood at 3.1 days, but levels of O type blood have fallen to below two days.
It aims to hold more than six days of blood in stock.
Hospitals have been told to protect their supplies, meaning non-urgent operations requiring blood could be postponed to ensure they are prioritised for patients who need them most.
Existing O negative and O positive donors have now been asked to book in appointments to give blood.
In the hours after the amber alert was announced, the Give Blood website appeared to be inundated with people wanting to book appointments.
People landing on the homepage were told they were in a "queue" before being able to choose a date and location for their donation.
O negative blood is the universal blood type that can be given to everyone. It is vitally important during emergencies and when the blood type of the recipient is unknown.
Blood can only be stored for 35 days, which means there is a constant need for donations - and a need for specific blood types.
The amber alert will last initially for four weeks, which NHSBT said should enable blood stocks to be rebuilt.
Around one in seven people have O negative blood.
Air ambulances and emergency response vehicles carry O negative supplies for emergencies.
Ongoing staffing issues are also thought to have contributed to the amber status, with more workers needed at donor sessions.
NHSBT said maintaining blood stocks has been a challenge after the pandemic, mainly due to staff shortages and sickness but also because people are less likely to visit collection centres in towns and cities.
The department will seek to tackle the issue by moving staff to the front line to open up appointments, speeding up recruitment to fill empty posts and using agency staff.
Wendy Clark, interim chief executive of NHSBT, said:
"Asking hospitals to limit their use of blood is not a step we take lightly. This is a vital measure to protect patients who need blood the most.
"Patients are our focus. I sincerely apologise to those patients who may see their surgery postponed because of this.
"With the support of hospitals and the measures we are taking to scale up collection capacity, we hope to be able to build stocks back to a more sustainable footing.
"We cannot do this without our amazing donors. If you are O positive or O negative in particular, please make an appointment to give blood as soon as you can. If you already have an appointment, please keep it."
You can register to give blood and book an appointment at www.blood.co.uk/
(c) Sky News 2022: First-ever amber alert issued by NHS over blood supplies as they fall critically low
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