A hospital is "working to manage" its oxygen supplies because of the rising number of COVID-19 patients needing treatment.
Southend Hospital in Essex said it was "experiencing high demand" for oxygen and appealed to the public to follow safety guidelines.
Yvonne Blucher, the hospital's managing director, said it was due to "rising numbers of inpatients with COVID-19 and we are working to manage this".
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She added: "The public can play their part by staying home and, where they cannot, following the 'hands, face, space' advice to cut the spread of the virus."
An internal document reported to have been sent to frontline NHS staff and seen by the BBC stated the hospital had "reached a critical situation with oxygen supply".
Mid and South Essex NHS Foundation Trust said the amount of oxygen used to treat patients should be reduced.
It said it was "imperative" supplies were used "safely and efficiently" and that all patients should have a target range for blood oxygen levels of 88-92%. Levels above 96% are deemed normal.
The document also said patients with a "saturation above 92% which are on oxygen should have their oxygen weaned within the target range".
Staff were assured the new guidance was safe and "no patient will come to harm as a result".
It comes as NHS England National Medical Director Stephen Powis told Sky News "the NHS is under intense pressure and that's potentially going to get worse".
He said the new vaccination centres being opened at seven UK sites were "another important step in the rollout of vaccinations", but that it would not have an impact on the pandemic "for a few months yet".
"It's really important that while we rollout the vaccines, everybody remembers to stick to those social distancing rules that are in place," he said.
"We all have a part to play in preventing deaths and we certainly have a part to play in stopping the virus being transmitted and taking the pressure off our hospitals."
