The UK on Sunday raised its Covid Alert Level due to the "rapid increase" in cases of the Omicron variant, as the government announced new measures to control its spread.
Prime Minister Boris Johnson, under pressure because of claims he and staff flouted Covid restrictions last Christmas, was due to make an address to the nation at 2000 GMT.
He is expected to talk about the country's booster programme.
The decision to increase the five-stage public health risk assessment from level three to four comes after a further 1,239 confirmed cases of the variant were recorded on Sunday.
That brings the total number of UK cases of Omicron to 3,137 -- a 65 percent increase from Saturday's total of 1,898.
Britain began easing coronavirus restrictions in June and the alert level was at stage three, which means the epidemic is in general circulation.
Level four means "transmission is high and pressure on healthcare services is widespread and substantial or rising".
The four chief medical officers for England, Wales, Scotland and Northern Ireland said the move was prompted by advice from the UK Health Security Agency, a public health body.
"Transmission of COVID-19 is already high in the community, mainly still driven by Delta, but the emergence of Omicron adds additional and rapidly increasing risk to the public and healthcare services," they said in a joint statement.
"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 officers said boosters were vital given that vaccine protection was reduced with Omicron, and both third jabs used -- Pfizer and Moderna -- increased immune response and showed "good effectiveness".
The move is designed to reduce already increased pressure on the state-run National Health Service (NHS) which is dealing with seasonal respiratory infections such as flu.
The government earlier announced additional measures to stop the spread of Omicron after facemasks were made compulsory in many indoor public places last Friday.
From Tuesday, fully vaccinated contacts of people who test positive for Covid-19 will be required to take daily lateral flow tests for seven days.
But those who have not had one or two shots of a Covid vaccine will have to self-isolate for 10 days, the Department of Health and Social Care said.
"The Omicron variant is quickly gaining ground in the UK and is expected to become the dominant strain by mid-December," said Health Secretary Sajid Javid.
"We are taking this proportionate and more practical measure to limit the impact on people's day-to-day lives while helping to reduce the spread of Omicron."
The government has set itself a target of giving everyone aged 18 and over a booster jab by the end of January and has opened up the programme to the over-30s from Tuesday.
The announcement adds to proposals from Johnson to combat Omicron, including a return to home-working if possible from Monday, and the introduction of vaccine passports in certain settings from Wednesday.
MPs vote to make the proposals law on Tuesday, with Johnson facing a potentially sizeable rebellion from his own Conservative colleagues.
The vote, though, is likely to pass with Labour support.
Britain -- one of the worst-hit countries by Covid-19 with more than 146,000 deaths -- began its mass vaccination campaign just over a year ago.
Infection rates remain stubbornly high at about 50,000 positive tests per day. In London, Omicron accounts for about a third of all Covid-19 cases.