Watford General Hospital’s oxygen system was running at such high capacity that bosses had no option but to take drastic action to ensure the safety of its patients.

A critical incident was declared at the hospital on Saturday when West Herts Hospitals NHS realised it needed to reduce the demand on the system.

The trust, responsible for Watford General, shut A&E to new patients and told them to find alternative hospitals.

Later on Saturday afternoon, the trust said it was dealing with a "technical issue with its oxygen equipment" but did not elaborate on what the "issue" was.

The critical incident was stood down at 10.30pm on Saturday.

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Now, the Observer can reveal what the issue was facing the trust on Saturday, which is linked to the coronavirus pandemic.

Trust chief executive Christine Allen said: "We didn’t run out of oxygen but our system was running at maximum capacity and so we needed to reduce the demand and make some adjustments to ensure it could continue to run safely."

Ms Allen added some patients were transferred to other hospitals and to ensure oxygen capacity was enough to meet the needs of existing patients, the hospital shut its doors to new admissions.

And Ms Allen has suggested this action may have to happen again as the coronavirus crisis continues.

Hertfordshire is right at the centre of the coronavirus epidemic in England, with 933 people testing positive for Covid-19 in the county as of today (Tuesday).

Only London, Birmingham, Hampshire, Essex, and Surrey have recorded more cases.

West Herts Hospitals NHS Trust has confirmed 40 deaths, with the current total in England at 5,655.

As of yesterday, there are around 170 patients with suspected or confirmed Covid-19 currently being treated at Watford General, and Ms Allen says the oxygen usage at the hospital has "doubled" over recent weeks.

Oxygen therapy via ventilators and continuous positive airway pressure (CPAP) machines are being used extensively to assist patients struggling to breathe because of Covid-19 – and this has been the case in Watford.

The Guardian reported on Sunday that the hospital trust was "considering alerting the military" that it may need help.

The paper has seen documents which stated healthcare planners in West Hertfordshire had "pleaded" with the government and NHS to "urgently approve" works to the hospital’s oxygen system, and warned NHS England last Wednesday that the situation was "critical".

The Guardian, which had spoken to a senior clinician, added the situation at Watford has become so "precarious", officials considered how to decide "who should receive the gas and who should miss out and likely die".

But the trust has told the Observer that none of the Covid-19 deaths at Watford General relate to issues with its oxygen system – and decisions on withdrawing breathing support are "entirely based on what is in patients’ best interests".

The trust added that despite declaring a critical incident, it never ran out of oxygen.

The trust declined to comment publicly on its communication with the Government.

NHS England was reported in the Guardian as saying it had alerted trusts in February, including West Hertfordshire, to the need to prepare oxygen systems.

An NHS spokesperson said: "All trusts were written to by NHS England in February reminding them to carry out engineering checks on ventilation equipment, and were further issued with guidance on how to use the kit safely, on April 1."

It has been reported that work to overhaul the oxygen system at Watford had been planned before the coronavirus outbreak, but had been put on hold.

The mayor of Watford, Peter Taylor, has said it was "shocking" that Watford General reached the stage that it had to close because the oxygen equipment was "not good enough".

He called for the Government to upgrade the system "now".

And Liberal Democrat leader Sir Ed Davey and former Watford parliamentary candidate Baroness Sal Brinton have piled the pressure on Health Secretary Matt Hancock by calling for a review of the NHS infrastructure available to provide oxygen in light of the incident in Watford.

Addressing the issue at a press briefing on Sunday, Mr Hancock said hospitals continued to have an "adequate" supply of oxygen as they deal with the pandemic.

Watford’s Conservative MP Dean Russell says he is in “daily contact” with the trust but said he had "not received any correspondence regarding concerns around the trust’s oxygen compressor".

He added: "As the MP for Watford I am here to support Watford General in any way that I can. Every single member of staff is a true hero."

Since declaring the critical incident, West Herts trust chief executive Ms Allen has said engineers continue to monitor the hospital’s oxygen usage "very closely" and a new manual system has been set up on top of the data provided by its equipment.

She added: "Engineers are working with us to boost our oxygen capacity and we are exploring mobile supplies – and have been for some time."

Ms Allen has also warned the hospital may have to close its doors again depending on the hospital’s oxygen usage.