The Omicron variant could trigger a surge in COVID infections bigger than previous waves in the UK with a risk it may overwhelm the NHS, a key group of scientists have warned.
Experts on the New and Emerging Respiratory Virus Threats Advisory Group (NERVTAG), which advises the government, held an extraordinary meeting last week to consider the new COVID variant, known scientifically as B.1.1.529, following its detection in South Africa.
According to a note of their meeting, which was observed by both a key Department of Health official and England's deputy chief medical officer Jonathan Van-Tam, the group concluded the introduction of Omicron to the UK "might have very serious consequences".
The note of last Thursday's meeting, published online on Monday night, stated that "if introduced into the UK, B.1.1.529 would likely be capable of initiating a new wave of infections".
"We cannot exclude that this wave would be of a magnitude similar, or even larger, than previous waves," they added.
"Although data on disease severity associated with B.1.1.529 are not yet available, a large wave of infections will be accompanied by a wave of severe cases and the subgroup cannot rule out that this may be sufficient to overwhelm NHS capacity."
NERVTAG called for "early and robust actions to prevent introduction and onward transmission".
This included action on early detection of Omicron cases in the UK, containment measures, an acceleration of the vaccine booster programme, a reassessment of the "optimal use" of antivirals, and preparations for modifications to existing vaccines and antibody treatments.
The group said that, although there was "current uncertainty" about the characteristics of Omicron, there were "sufficiently worrying signals" for them to provide their advice.
Their initial conclusions, shared with government officials, are based largely on what is known from cases in South Africa.
The NERVTAG meeting, held via telephone conference on Thursday afternoon, came the day after South African authorities first reported the discovery of the Omicron variant.
The UK government added six southern African countries, including South Africa, to its travel red list on Friday.
A further four countries were added over the weekend.
At a news conference on Saturday, Prime Minister Boris Johnson said it "does appear that Omicron spreads very rapidly and can be spread between people who are double vaccinated" and the current vaccines "might" be less effective against the variant.
Mr Johnson has reimposed compulsory mask-wearing in England in shops and on public transport, ordered people to self-isolate if they have been a contact of someone infected with the Omicron variant, reintroduced day two PCR testing for all people arriving into the UK from abroad, and extended the rollout of booster jabs.
But the prime minister has also said the UK is in an "immeasurably better" position to deal with COVID than it was a year ago.
There have now been 22 confirmed cases of the Omicron, with Health Secretary Sajid Javid telling Sky News that number would "certainly go up".
However, Mr Javid also said "a lot more" is to be yet learned about Omicron with scientists still to determine whether it is any more dangerous than the Delta variant, which is the current dominant strain of coronavirus in the UK.
He added he did not "anticipate" any further restrictions being brought in due to the Omicron variant, beyond those the government has already introduced.
Government sources said they had already taken NERVTAG's advice on board with their action on masks, boosters, self-isolation rules, and travel restrictions.
They also said that NERVTAG's warning they could not rule out a new wave of infections overwhelming the NHS was not the same as it being a "likely" scenario.
The NERVTAG briefing does not call on the government to introduce further restrictions than are now in place at this stage.
Meanwhile, it was reported that leaked minutes of a meeting of the Scientific Group of Emergencies held on Monday said that officials should prepare now for a "potentially significant" wave of infections while data on Omicron is analysed.
The BBC said that minutes of the meeting added that it is "highly likely" that Omicron can escape immunity caused by previous infection or vaccination "to some extent".
A government spokesperson said: "As soon as we became aware of the Omicron variant we took rapid action to protect public health - including expanding the red list, introducing PCR testing for travellers, changing the rules on self-isolation for close contacts and mandating face coverings in shops and on public transport.
"On top of this we will be offering everyone over 18 a booster jab, as well as second jabs for 12-15 year olds, following advice from the JCVI.
"We continue to monitor the situation closely and will not hesitate to take further action if necessary."
Two of the three biggest vaccine manufacturers have tried to calm fears about the new strain.
Both the University of Oxford, which is behind AstraZeneca vaccine, and Pfizer-BioNtech have predicted existing jabs would continue to prevent severe disease.
On Monday, the chief executive of Moderna, Stephane Bancel, caused jitters after telling the Financial Times that existing vaccines would be much less effective at tackling Omicron than earlier strains - and that it would take months for pharmaceutical companies to manufacture new vaccines.