A hospital's oxygen supply has "reached a critical situation" due to rising numbers of Covid-19 infections.

A document shared with the BBC showed Southend Hospital has had to reduce the amount it uses to treat patients.

It said the target range for oxygen levels that should be in patients' blood had been cut from 92% to a baseline of 88-92%.

"We are experiencing high demand for oxygen because of rising numbers of inpatients with Covid-19 and we are working to manage this," she said.

In the document, from the Mid and South Essex Hospitals Foundation Trust, which has been shared with frontline NHS staff, the oxygen supply was said to have "reached a critical situation".

It said it was "imperative we use oxygen efficiently and safely" and states patients who are being fed oxygen and have an oxygen saturation of above 92% "should have their oxygen weaned within the target range", which is now 88-92%.

GPs in Essex have told the BBC that the threshold for sending a patient to hospital for supplemental oxygen is if their oxygen saturation is at 92%.

Chris Hopson, chief executive of NHS Providers which represents hospital trusts in England, said there was "huge pressure" on hospital oxygen stocks because giving patients extra oxygen was a "key part" of coronavirus treatment.

"If you [a hospital] push your oxygen to an absolutely critical level, then the thing that you can't do is have the oxygen system break down...

so effectively you will have to dial it down, in which case you will probably have to transfer patients to the nearest neighbouring hospital for a short period of time.

"I cannot tell you how much work has been done over the summer and autumn to ensure that people [hospital trusts] have been prepared for this...

Many people who are sick with Covid will need extra oxygen to help them breathe.

As Covid admissions increase, it can put huge demand on a hospital's piped oxygen supply system to provide this high flow.

When stretched to the maximum, other steps are needed, such transferring patients elsewhere or limiting how much oxygen is pumped to each patient.