The UK’s chief medical officers and NHS England’s national medical director made the recommendation to ministers.
The UK Covid alert level has been raised following a rapid increase in Omicron cases being recorded.
The country’s four chief medical officers and NHS England’s national medical director have recommended to ministers that the UK go up to Level 4 from Level 3.
It comes as Boris Johnson prepares to address the nation at 8pm about the booster vaccine programme.
The message – which PA news agency understands will not see new restrictions announced – will be pre-recorded, meaning the Prime Minister will be able to avoid answering questions about suggestions he broke coronavirus rules by attending a virtual Downing Street staff Christmas quiz last December.
The decision to increase the alert level follows advice from the UK Health Security Agency (UKHSA) after a further 1,239 confirmed cases of the Omicron mutation were recorded in the UK as of Sunday.
It brings the total number of UK cases of Omicron to 3,137, a 65% increase from Saturday’s total of 1,898 UK cases.
Increasing the UK Covid alert level to Level 4 means the epidemic is “in general circulation, transmission is high and direct Covid-19 pressure on healthcare services is widespread and substantial or rising”, according to Government guidance.
In a joint statement, the CMOs and NHS England’s Professor Stephen Powis said the emergence of Omicron “adds additional and rapidly increasing risk to the public and healthcare services”.
They added: “Early evidence shows that Omicron is spreading much faster than Delta and that vaccine protection against symptomatic disease from Omicron is reduced.
“Data on severity will become clearer over the coming weeks but hospitalisations from Omicron are already occurring and these are likely to increase rapidly.”
The five officials, including England’s CMO Professor Chris Whitty, Northern Ireland’s Sir Michael McBride, Scotland’s Professor Gregor Smith, and Wales’ Dr Frank Atherton, said the NHS was already under pressure “mainly driven by non-Covid pressures”, with Omicron’s ability to escape vaccines “likely” to add to those demands.
“It is extremely important that if you are eligible, you get your Covid vaccination now – whether this be your first, second or booster dose,” they said.
“People should continue to take sensible precautions including ventilating rooms, using face coverings, testing regularly and isolating when symptomatic.”
The development comes as it was warned more restrictions may be needed to tackle Omicron, with the UK facing an “inevitable” large wave of infections.
That is according to Dr Susan Hopkins, the chief medical adviser for the UKHSA, who said people will have to reduce social contact as much as possible, with Omicron cases being detected among hospital patients – although no deaths have yet been reported as a result of contracting it.
Dr Hopkins said more measures may be needed, adding that the Government has “very difficult” decisions ahead, even after triggering its winter Plan B proposals, which include introducing Covid passports for large venues.
“I think that the restrictions that the Government have announced are sensible. I think that we may need to go beyond them. But we’ll need to watch carefully what happens with hospitalisations,” she told the BBC.
Education Secretary and former vaccines minister Nadhim Zahawi described the UK as in a “race” to get the coronavirus booster to eligible adults “as quickly as possible”.
New modelling suggests that, under one scenario, almost twice the number of coronavirus patients could be admitted to hospital compared with last year due to the impact of Omicron.
Experts from the London School of Hygiene & Tropical Medicine (LSHTM) calculated figures which say a large wave of infections could occur over the next few months if tougher measures are not brought in.
Analysis by the UKHSA has found that the AstraZeneca and Pfizer vaccines provide “much lower” levels of protection against symptomatic infection with Omicron compared with Delta.
But the UKHSA said a booster dose gives around 70% to 75% protection against symptomatic infection with Omicron, as it urged people to have their boosters.
All eligible adults can book their booster jab two months (61 days) after their second dose using the NHS national booking system, getting their top-up in protection three months (91 days) on from their second dose.
The national booking system will open to everyone aged 30 to 39 in England from Monday.
Meanwhile, double-jabbed people identified as a contact of someone with Covid-19 in England will be told to take a daily rapid test for seven days from Tuesday.
Unvaccinated adults are not eligible for the new daily testing policy and they must self-isolate for 10 days if they are a contact of someone who tests positive.