An NHS hospital in Norwich has had to close four beds in its high dependency unit because it does not have enough nurses to staff them.

Norfolk and Norwich university hospital (NNUH) decided on Monday to temporarily shut the beds in the Gissing ward of its critical care complex.

The beds, which are used for seriously ill patients, have been shut despite flu and other viruses that circulate at this time of year leaving more patients than usual suffering from breathing problems.

Hospital managers told doctors in an email that: “A decision has been made to temporarily close our GHDU beds and reduce to 20 bed capacity on our CCC [critical care complex] from today as the nursing staffing is insufficient to keep Gissing open.”

A spokesman for the Norwich hospital confirmed later: “We have temporarily reduced the number of high dependency beds at NNUH from 24 to 20 in order to maintain national intensive care staffing standards. During this time the HDU area on Gissing ward will be repurposed for general surgical patients. We are actively recruiting specialist staff to reopen those four beds for HDU as soon as possible.”

The closure is another stark illustration of both the lack of staff in the NHS, which in England has around 100,000 vacancies, and the extra strain winter is putting on hospitals.

The Norfolk and Norwich has come under particular pressure in recent weeks. The Guardian recently disclosed that it was so overwhelmed by demand that it had told senior doctors to make “the least unsafe decision” when treating patients.

NHS bosses warned recently that staff shortages were now so widespread that patients’ safety and quality of care are under threat.

Derriford hospital in Plymouth has also come under intense pressure in a winter that has seen more cases than usual of both flu and norovirus, the winter vomiting bug.

It told staff on Monday that the sheer weight of demand for care meant it had started the day “at 104% [bed] occupancy” and on “black alert” – an official admission by an NHS trust that it is struggling to cope.

It added: “We recognise teams are stretched with outliers and appreciate the efforts needed to ensure ward rounds go ahead as soon as possible.”

Dr Sue Crossland, the president of the Society for Acute Medicine, said lack of social care support over the festive season was partly to blame for the “extreme stress on the system”.

“Every acute trust I know is suffering with overwhelming demand for beds, which are all full. This is putting pressure on emergency departments, which are getting towards standstill, and acute medical units, which are trying to help absorb the flow into hospital.

“The demand for admission is overwhelming and there is so little movement due to social care being less accessible over the extended holiday period. Patients are sick, lots are requiring admission but beds are full with both sick and waiting for social care patients,” Crossland said.

Dr Julia Patterson, the lead for EveryDoctor, an organisation of grassroots doctors, said: “It is absolutely appalling that adult high dependency unit beds are being closed in an NHS hospital due to unsafe staffing levels. These beds are used by patients requiring a high level of care from skilled professionals. These patients now won’t receive that specialised care, and they will suffer for it.

“The current situation – hospitals on black alert up and down the country, cancelled cancer operations, closed HDU beds – highlights that the government’s NHS austerity cuts have gone too far, and patients’ lives are now at risk because NHS trusts do not have the resources to care safely for patients”.

In a message, managers at Southend hospital thanked staff for their hard work over Christmas but added that: “Not surprisingly [we] have tipped into [an] internal critical [incident] this morning” because of the large number of patients it had decided to admit but did not have a bed for.”

One doctor at Southend, who asked not to be named, said: “The situation in A&E is unsafe every day due to there being nowhere for patients waiting for a hospital bed. This results in nowhere to see new patients, including at times the inability to receive blue [light] emergency calls into the resuscitation room.”

Hospitals have taken steps to prepare for winter, including opening “escalation” or overflow wards in which to look after patients, paying for some patients who are nearing discharge to be moved to a care home and installing temporary portable wards in their grounds.

An NHS spokesperson said: “Hospitals have more beds open than this time last year, but flu and norovirus are adding pressure at a time when the NHS is already looking after significantly more people than ever before. The NHS has already looked after a million more people in A&E this year compared to last, and at this particularly busy time it’s really important that the public help our hardworking staff by getting their flu vaccine now, using the free NHS 111 phone and online service for urgent medical needs, and seeing their local pharmacist for minor ailments before they get worse.”