The government has been warned it must be ready to apply restrictions "at pace" amid fears the NHS could be overwhelmed by the rapid spread of the Covid-19 Omicron variant.
The health service is setting up "surge hubs" at hospitals across England to cope with rising patient numbers.
Hospitals have already been forced to use hotels, hospices and care homes to safely discharge as many people who are medically fit to leave as possible.
Chris Hopson, chief executive of NHS Providers, said trust leaders recognise that the UK government’s threshold for introducing extra measures in England "hasn’t been crossed yet" but that additional capacity is being created in case hospital pressures increase.
The number of coronavirus patients in UK hospitals has jumped by more than 40% in a week to the highest number since March 2, according to government data up to December 29.
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It came as a leading scientist said it is likely that the NHS will be overwhelmed by the spread of Omicron.
Professor Peter Openshaw, who sits on the New and Emerging Respiratory Virus Threats Advisory Group (Nervtag), told BBC Breakfast: "I think we haven’t quite reached the threshold that was set by government in terms of the NHS being overwhelmed, but it looks like that will be reached quite quickly.
"What I’m very concerned about is our NHS staff, my dear colleagues who have worked so, so hard all through the repeated waves of this infection. How are they going to cope?"
In England alone, the number of patients in hospital has climbed to its highest level since February, but at that time the country was under a strict lockdown.
Mr Hopson warned even if extra restrictions are put in place to control the Omicron variant, it will take two weeks to reduce the hospital admission rate.
He told BBC Radio 4’s Today programme that additional restrictions "may be needed at pace if the evidence warrants it".
"Trust leaders can see why the government is arguing that, in the absence of a surge of seriously ill older patients coming into hospital, that threshold hasn’t yet been crossed," he said.
"But we still don’t know if a surge will come, and indeed we are exactly talking about the preparations we are making for that surge right now.
"So, in terms of restrictions, I think we are in exactly the same place we’ve been for the past fortnight, which is the government needs to be ready to introduce tighter restrictions at real speed should they be needed."
Dr Chaand Nagpaul, chair of the council at the British Medical Association (BMA), said there is concern over the "significant increase" of people in hospital with the virus.
Speaking to Sky News, he said: "While the proportion of people who end up in hospital as a result of Omicron is smaller, we are definitely seeing significant increases."
The BMA has also warned of issues with the supply of Covid tests meaning more and more NHS staff could soon be off work.
The group has urged the government to "do everything possible to ensure that enough tests are available for key workers as a priority," pointing to 18,000 staff absences across acute hospital trusts in England.
Dr Nagpaul said 25,000 patients were admitted to hospital with Covid-19 in a four-week period before Christmas, but added that he does not have the data on whether all of those were being treated for their symptoms or were admitted with other ailments before testing positive.
Separate data on hospital admissions suggests that 71% of Covid patients in hospitals in England on December 21 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.
More up-to-date figures are expected from NHS England on Friday.
In Scotland and Wales, nightclubs are closed to New Year’s Eve partygoers, and restrictions have been placed on hospitality.
In Northern Ireland, nightclubs have closed and there's a ban on dancing at hospitality venues while table services has returned in bars and restaurants as well as two-metre social distancing in offices.