Omicron is causing around 200,000 new infections a day, modelling from the UK Health Security Agency has suggested.
The figure was revealed by health secretary Sajid Javid in the House of Commons on Monday, and comes as the UK recorded its first death involving Omicron.
Some 10 people are in hospital with the variant. Most of these 10 have received two vaccines and range in age from 18 to 85, though there are no details on whether they have underlying conditions.
The figures underline why there’s urgency to expand the UK’s booster vaccine programme, and Javid said the NHS will need to exceed 840,000 booster jabs per day in a bid to fight Omicron.
The Cabinet minister told MPs that while there were now 4,713 confirmed cases of the variant in the UK, the UK Health Security Agency (UKHSA) estimates that the current number of “daily infections are around 200,000”.
He added: “While Omicron represents over 20% of cases in England, we’ve already seen it rise to over 44% in London and we expect it to become the dominant Covid 19 variant in the capital in the next 48 hours.”
Overall, there were a further 54,661 lab-confirmed Covid-19 cases in the UK as of Monday morning, the government said.
It follows confusion over whether the government has promised that people can all have a jab in their arm by the December 31 deadline, or whether they will just have an offer of a future vaccine.
Javid said every adult across England could expect to be offered a “chance to get boosted by the end of this month” – though he suggested not everyone would get a dose in December.
He said: “It is asking a huge amount of our colleagues in the NHS.
“And it’s our joint view that we can try to offer adults a chance to get boosted by the end of this month.
“And that does not mean every single person necessarily can get that booster, it requires them to come forward and to take up this offer as well, as well as everything going right in this huge expansion plan.”
Javid said he acknowledge that “our national mission comes with some difficult trade-offs”, meaning some non-urgent appointments and surgery in the NHS may be cancelled.
Earlier, Boris Johnson announced the first UK death with Omicron during a visit to a vaccination clinic near Paddington in west London, as he repeatedly declined to rule out further coronavirus restrictions ahead of Christmas but stressed the urgency of people getting boosters.
“Throughout the pandemic I’ve been at great pains to stress to the public that we have to watch where the pandemic is going and we take whatever steps are necessary to protect public health,” he added.
In England a booster is available to everyone aged 18 or over from this week as long as the second dose was at least three months ago.
Over-30s can already book a booster online and, from Wednesday, this will be extended to over-18s.
Johnson said MPs thinking of rebelling against Plan B measures needed to recognise there was “no room for complacency” in dealing with Omicron.
Responding to the fact the government website said on Monday “there are no more home tests available” when people tried to order lateral flow kits, the prime minister said there was a “ready supply” of tests.
UKHSA said earlier that “due to exceptionally high demand, ordering lateral flow tests on gov.uk has been temporarily suspended to fulfil existing orders”.
It added: “Everyone who needs a lateral flow test can collect test kits, either at their local pharmacy, some community sites and some schools and colleges.”