More than 20 NHS trusts have now declared a critical incident as the number of people in hospital with COVID hits the highest level since last February, amid rising pressure on health services from the surge in Omicron cases.
It comes as Chris Hopson, the head of NHS Providers, which represents trusts, warned coronavirus hospitalisations were "rapidly increasing outside London".
He tweeted that while the capital had the "lowest weekly growth rates", infections were now "fully hitting [the] older population and we still don't know the full effect of Xmas/New Year mixing & school return".
As of 4 January, a total of 17,276 people were in hospital in the UK with coronavirus. In comparison, during the second wave, the numbers in hospital peaked at 39,254 on 18 January 2021.
'Hospital admissions double every nine days'
Boris Johnson said on Wednesday that hospital admissions were "doubling around every nine days" and the country was "experiencing the fastest growth in COVID cases we've ever known".
Staff shortages due to self-isolation are also hitting the NHS. The quarantine period for people in Scotland is now being cut from 10 to seven days, bringing it broadly in line with the rest of the UK.
Another 194,747 coronavirus cases and 334 deaths have been reported in the latest daily UK figures.
The number of fatalities - the highest reported since 2 March - includes a backlog of some data from NHS England stretching back to New Year's Day.
No 10 said more than 20 trusts had reached the alert level where priority services may be under threat, but stressed it was "not a good indicator" of the pressures the health service was under.
The prime minister's official spokesman said: "It's worth understanding that critical incidents can last, in some certain circumstances, a matter of hours, a morning, or afternoon, a day… some of them can last longer than that."
But Mr Hopson said that declaring a critical incident is "an indication of serious pressure".
He added that it allowed trusts to "clearly flag an impending problem", seek help from staff and other agencies, and make swift decisions to redeploy staff or reprioritise services.
NHS 'stretched like never before'
Mr Hopson told Sky News that absences are having a "massive impact" on the NHS, which is "really struggling in some places".
And he warned on Twitter that the health service is being "stretched like never before".
He wrote: "Pressure shifting from London to the rest of the country and rising significantly, at pace. Combination of three things at once: rising COVID hospitalisations and growing staff absences on top of large non-COVID pressure."
Mr Hopson continued: "Today's COVID hospitalisation data (5/1/22) shows COVID-19 hospitalisations rapidly increasing outside London. Over last seven days, hospitalisations up 85% in NE/Yorks. 82% in NW. 61% in Midlands. 52% in East of England. 40% in South West. 33% in South East. 23% in London."
He added that he had spoken to more than a dozen NHS trust bosses outside London over the last 24 hours. He said they had all "challenged the assumption" that the rest of the UK will follow the capital in the "shape and scale of, and ability to 'cope' with, pressure".
Data last month suggested 71% of COVID patients in hospitals in England were primarily being treated for the virus.
The remaining 29% were there "with COVID", suggesting they tested positive on arrival for another ailment or tested positive during their stay. Some medics are calling this latter group incidental COVID patients.
One of the most recent organisations to declare a critical incident was Norfolk and Waveney Clinical Commissioning Group.
In Norfolk and Waveney, the three main hospitals are Norfolk and Norwich University Hospital NHS Foundation Trust, James Paget University Hospitals NHS Foundation Trust and the Queen Elizabeth Hospital King's Lynn NHS Foundation Trust.
Other critical incidents have been declared by Morecambe Bay NHS Trust, Blackpool Teaching Hospitals NHS Trust, University Hospitals Plymouth NHS Trust, United Lincolnshire Hospitals NHS Trust, Gloucestershire Hospitals NHS Foundation Trust and Great Western Hospitals NHS Foundation Trust.