The NHS faces intensifying strain over coming weeks as a "health emergency" created by pressures such as surging hospital admissions and staff absences build further, it has been warned.
Matthew Taylor, head of the NHS Confederation, which represents organisations in the healthcare system in England, Wales and Northern Ireland, acknowledged the current Omicron-driven wave of COVID cases looked "less bad" than previous waves, with apparently milder symptoms.
But he suggested that if a doubling in hospital admissions seen over the last two or three weeks continues over coming weeks, the stress on the system would build further.
"We would then be a situation where it would be just very hard for the health service to do anything else than deal with coronavirus," Mr Taylor told Sky News.
Hospital admissions in England have risen to their highest level since January 2021, while the number of NHS hospital staff absent due to the virus nearly doubled in a month.
And in Wales, one hospital has told people to stay away from A&E.
Swansea Bay NHS trust tweeted: "A staff shortage worsened by COVID means we can only provide a limited service at Morriston ED over the bank holiday weekend. ED is for life-threatening illness or serious injury ONLY. Please use alternatives whenever possible."
Government figures show there were 12,395 people in hospital with COVID in England on New Year's Eve - the highest number since 25 February last year, when there were 12,449 hospitalisations.
Another top health official, NHS Providers chief executive Chris Hopson, told Sky News that the NHS was already "beyond full stretch" and warned that the knock-on effects of different generations mixing over Christmas could see it come under "extreme pressure".
Mr Hopson stressed that if hospital admissions reach a point where the NHS "is not going to be able to treat the people who need to be treated then obviously the issue of further restrictions will need to come back on the agenda".
The comments from the NHS leaders came after Health Secretary Sajid Javid said in a newspaper article that further restrictions triggered by the pandemic must be an "absolute last resort" and that the UK would have to "live alongside" the virus in 2022.
His remarks were published a day after figures showed rising infection numbers across the UK, with the rate of infection at one in 25 in England and one in 15 in London, in the week to 23 December.
Mr Taylor echoed Mr Javid by saying that "in the medium term we should be very hopeful" and that this year should reach a stage where COVID is no longer "something which has to disrupt our day to day lives".
But he added: "That doesn't take away from the really acute challenges facing the health service now, challenges which are absolutely bound to continue for several weeks to come."
Mr Taylor said the NHS was heading for its traditional mid-January peak while facing the added strain of waiting lists that have been building over the last couple of years as well as the latest coronavirus surge - causing higher admissions while also adding to staff absences.
Such pressures have created a situation where some hospital managers have been cancelling visiting, itself a last resort for the health service, he added.
"I don't think anyone is denying that we are in anything other than a health emergency right now," Mr Taylor said.
"We cope in difficult circumstances and we'll carry on doing that.
"But if these numbers continue to rise then every day that becomes more difficult to do."
Mr Hopson said that the NHS was currently "absolutely flat out" with emergency cases, surgery that cannot be delayed any longer, and the vaccine booster programme.
He said that in London, which has borne the brunt of the Omicron wave, growth in COVID-19 hospital admissions had slowed over the last few days.
"But what we are very worried about is that if we see the effect of intergenerational mixing over Christmas you could see those rates go back up again - and if they go back up again the NHS really will come under extreme pressure," Mr Hopson added.