Doctors have made an urgent plea to keep Christmas celebrations small as scientists said mounting evidence that the Omicron variant of coronavirus is milder may not be enough to stop the NHS being overwhelmed.

In the first government study of its kind, the UK Health Security Agency (UKHSA) found that the risk of being admitted to hospital is up to 70% less for people with Omicron compared with those infected with Delta.

But the Scientific Advisory Group for Emergencies (Sage) warned ministers that the new variant would have to be about 90% less severe for hospital admissions not to reach previous peak levels, “unless the wave peaks early for other reasons”, according to minutes published on Thursday.

The health secretary, Sajid Javid, welcomed the UKHSA findings as “promising” but said cases were rising at an “extraordinary rate” and it was still “too early to determine next steps”.

The Scottish government announced on Thursday that nightclubs would close for three weeks from 27 December, the latest in a series of Covid curbs introduced by the devolved governments in Scotland, Wales and Northern Ireland. The UK government has said no further measures in England will be announced before Christmas.

Official data on Thursday showed there were 119,789 new Covid infections across the UK in the previous 24 hours – a record number – meaning more than 600,000 people will be in isolation on Christmas Day. A further 147 coronavirus deaths were recorded on Thursday, with daily deaths relatively stable.

With millions of Britons now already with their Christmas companions or travelling to spend the weekend with family or friends, Dr Nick Scriven, a past president of the Society for Acute Medicine, told the Guardian: “I personally would advocate caution, especially as the Omicron variant seems so transmissible, even if only for a few days.

“What I mean by this is doing lateral flow tests before any gatherings and perhaps limiting the numbers people mix with over the next few days. [And] if you are not fully vaccinated with a booster, I would be very cautious about mixing outside your family circle.”

In a pointed reminder of revelations of rule-breaking get-togethers involving Downing Street last Christmas, Scriven, a senior doctor in Yorkshire, said: “We are really relying on people doing what experts suggest and not using politicians as role models.”

Dr Andrew Goddard, the president of the Royal College of Physicians, acknowledged that while people may find it hard to avoid being in a large group, they should do so to help limit soaring sickness absence among NHS staff.

“While we are all looking forward to time with loved ones this year, we need to think very carefully about the number of people we mix with over the next couple of weeks. That’s not a message people want to hear and not one that’s easy to deliver. But if we aren’t cautious, we risk the number of available health and care staff falling to very dangerous levels,” Goddard said.

NHS England data released on Thursday showed that the number of health service staff days lost to Covid in England soared by 38% last week, to 124,855. Hospital bosses privately voiced deep unease that the sharp rise, coming on top of the service’s almost 100,000 vacancies, could hinder its ability to cope with a feared wave of hospitalisations driven by the new variant.

Prof Stephen Powis, NHS England’s national medical director, declared that the NHS was “on a war footing”. Matthew Taylor, the chief executive of the NHS Confederation, said the service was facing “a double emergency” of a growing number of staff off sick at the same time as hospitalisations because of the virus were rising sharply.

Chris Hopson, the chief executive of NHS Providers, cautioned that despite Omicron being milder, the health service could still come under very serious pressure.

“We still don’t have conclusive data in terms of what impact Omicron could have in terms of level of hospitalisations. We have some very early studies, but it’s a bit like trying to predict the result of a football game when you’re only a third of the way through,” he told BBC Radio 4’s Today programme.

The leader of Britain’s family doctors also advised restraint in personal behaviour in the face of Delta and Omicron fuelling a new surge in Covid cases. “Even if vaccinated, it’s important that people behave cautiously while new cases remain high,” said Prof Martin Marshall, the chair of the Royal College of GPs.

As well as wearing a mask, washing their hands and practising social distancing, people should be prepared to isolate over Christmas if they test positive in order to not spread Covid to others, he added.

According to the UKHSA research, people with Omicron are between 31% and 45% less likely to go to A&E compared with those with Delta, and 50% to 70% less likely to require admission to hospital. The early findings are consistent with analysis published on Wednesday by Imperial College London and the University of Edinburgh.

Lockdown-sceptic Conservatives have seized on the analysis as fresh evidence to support the idea that more restrictions are unnecessary.

“It does look like it’s less severe,” said Mark Harper, the chair of the backbench Covid Recovery Group. “I just don’t buy the idea you’re going to have immense numbers of people in hospital, given that most people have been boosted now. You might want to have some guidance, but I can’t see much of a case for restrictions, to be honest.”

Harper was among the 100 Conservative MPs who voted against the introduction of compulsory Covid passes this month.

Boris Johnson has promised to recall parliament if he decides stricter rules are needed in the coming days, but he would be likely to face a fierce backlash from his own MPs, including in cabinet.

One minister told the Guardian they believed the UKHSA analysis, together with “people’s self-control” and higher immunity from the booster campaign, supported the idea that “draconian” restrictions could be avoided.