Hospitals have been instructed to ration oxygen due to the pressure on supplies from coronavirus patients, according to reports.
Under new guidance, doctors have been told to lower their targets for how much oxygen sick people should have in their blood to 92 to 96%, the Daily Telegraph reports.
The target saturation range is normally 94 to 98%.
But as the coronavirus crisis reaches its peak in the UK, medics have even been told to consider a range of 90 to 94% "if clinically appropriate", to save the vital gas for those who need it the most.
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Hospital bosses are understood to have been told of the new rules, which apply to all patients and not just those who have Covid-19, on April 9.
Hypoxia, when all or part of the body does not have adequate oxygen supply, can lead to organ failure and brain damage.
The new targets were issued just days after Watford General Hospital was forced to turn patients away as their oxygen system hit maximum capacity due to too many Covid-19 patients.
The hospital declared a critical incident on Saturday April 4 and said they needed to 'reduce the demand' so they could continue to run safely.
Patients were urged to use other hospitals with A&E departments for emergencies.
Dr Graham Burns of the British Thoracic Society told the Telegraph that the new target saturation levels are "not ideal".
“90 per cent is when you start to approach the cliff face, so we prefer 94 per cent because it’s a step away from that,” he said.
“Where hospitals are fortunate enough to be delivering adequate flow, then a higher level would be better.”
It is understood that although the UK has good supplies of medical oxygen, only a limited volume can be pumped around hospitals at one time.
NHS England says it has been providing engineering support and advice to trusts across the UK to ensure supplies can meet the surge in demand.
Dr Ramani Moonesinghe, the national clinical director for critical care for NHS England and Improvement said: “Hospital oxygen tanks are fuller than normal, having been filled more frequently in recent weeks as part of preparations for patients with coronavirus, so there is sufficient oxygen supply available.
"The revised clinical guidelines, agreed with all relevant clinical experts, will help local teams manage patient care safely in the context of the higher than usual number of people who are receiving oxygen treatment.”
The UK's coronavirus death toll has increased to 14,653 after another 825 hospital deaths - a drop from the previous daily total.
NHS England reported a further 738 deaths on Friday as experts say there are signs the UK is reaching its peak and the curve is flattening.
Scotland reported 58 new deaths as First Minister Nicola Sturgeon said lockdown restrictions have slowed down the spread of Covid-19, calling it "the first signs of hope".
A further 18 deaths have been confirmed in Northern Ireland and 11 in Wales.
The total stood at 13,828 at the same time on Thursday after a rise of 870.
