A further 86 cases of the new Omicron COVID variant have been reported in the UK, taking the total to 246.
This compares with a total of 160 yesterday - an increase of more than 50%.
The UK Health Security Agency (UKHSA), which publishes the figures, says 18 of the cases are in Scotland, taking its total up to 48.
Meanwhile, another 43,992 cases of coronavirus have been recorded in the UK compared to 42,848 on Saturday.
There were another 54 deaths within 28 days of a positive test for the virus.
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Separate figures published by the Office for National Statistics show there have now been 170,000 deaths registered in the UK where COVID-19 was mentioned on the death certificate.
A total of 51,094,640 first vaccine doses had been delivered in the UK by 4 December, Government figures show. This is a rise of 25,679 on the previous day.
Some 46,527,302 second doses have been delivered, an increase of 35,719.
A combined total of 20,258,417 booster and third doses have also been given, a day-on-day rise of 448,975.
Separate totals for booster and third doses are not available.
The figures come as one expert told Sky News the UK could face "more stringent measures" after Christmas to counter the spread of the new variant.
Professor Sir David Spiegelhalter said the emergence of Omicron in the UK was "appalling timing" as the next few weeks are "one of the highest risk periods... with a lot of indoor socialising".
He said that the government "would love not to disrupt Christmas", but he warned: "What happens after Christmas is another matter."
Although the UK is still waiting for real-world data on the new variant, Professor Spiegelhalter, a statistician at the University of Cambridge, said that currently it does not appear to have as much of a risk of severe disease.
And he also praised the booster vaccines for "saving very large numbers of lives".
Deputy Prime Minister Dominic Raab also told Sky News, "the government wants people to enjoy Christmas this year", adding that employers should take a "common sense" approach to office parties.
The government has already reintroduced several measures to tackle Omicron, including mandatory face masks in shops and on public transport, and mandatory pre-departure testing for all UK arrivals.
Bringing back pre-departure testing came after analysis by the UKHSA suggested Omicron could narrow the window between infection and infectiousness compared to previous strains.
There are warnings the NHS will be in a "very, very difficult position" if the Omicron variant were to lead to a surge in hospital admissions.
The president of the Royal College of Emergency Medicine, Dr Katherine Henderson, said hospitals were already struggling to cope as they enter winter.
"It is pretty spectacularly bad now, it will get worse - and if the new variant becomes a thing in terms of numbers and translates into hospitals admissions we are going to be in a very, very difficult position," she said.
"We will always still be there. We still want patients to come, but we do have to help people to understand that really at the moment the service is so stretched that an extra push could be very, very difficult."
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