At least one person in the UK has died with the Omicron variant of Covid, the prime minister has confirmed - and some 4731 cases have now been confirmed.
Speaking during a visit to a vaccination clinic near Paddington, in west London, Boris Johnson said: "Sadly yes Omicron is producing hospitalisations and sadly at least one patient has been confirmed to have died with Omicron.
"So I think the idea that this is somehow a milder version of the virus, I think that's something we need to set on one side and just recognise the sheer pace at which it accelerates through the population. So the best thing we can do is all get our boosters."
He was also unable to rule out any new coronavirus restrictions being introduced before Christmas.
Amid repeated questioning, he said: "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."
Sajid Javid warned the UK is in a race against the variant after Boris Johnson issued a plea to the nation on Sunday evening, warning the public of a "tidal wave of Omicron" that could cause "very many deaths".
Updating MPs, Mr Javid said there is estimated to be 200,000 new daily cases of Covid, with the Omicron expected to become dominant in London within 48 hours.
There are currently at least 10 people in hospital with Omicron, and at least one of those is just 18 years old, according to the UK Health Security Agency.
The health secretary said the government is aiming to vaccinate a million people every day, starting this week, if the UK is to reach another ambitious target of having offered all eligible over 18s a booster jab before the end of the year.
But achieving that number - higher than all other previous vaccination records - will take a huge effort, with health workers facing an “incredibly difficult” challenge, according to a boss at the NHS.
Saffron Cordery, deputy chief executive of the NHS Providers organisation, said the public will need to "understand that waiting times might be longer, that routine care might not come at the time that they were expecting it".
“And also unfortunately that, for example, operations may be cancelled and rescheduled, so it’s a really difficult situation. It’s a really challenging situation for the NHS front line,” she told the BBC.
Downing Street said cancer treatment would not be "degraded" but accepted there would be a "level of disruption to non-urgent care".
It comes with the NHS waiting list currently at its highest ever, with more than six million people awaiting treatment in England.
Mr Javid told ITV News that Omicron "spreads at a phenomenal rate, the number of infections is doubling every two to three days, and that means we're in a new race, whether we like it or not, between the virus and the vaccine".
The PM said the variant now represents about 40% of coronavirus cases in London and "tomorrow it'll be the majority of the cases" in the capital.
But no new Covid restrictions are being planned, Mr Javid said, aside from the new requirement of proving Covid status before entering large venues which will be introduced on Wednesday, however he encouraged everyone to test themselves before attending gatherings.
But the government website for ordering tests currently says "there are no more home tests available", telling people they should "try again later".
Despite the message, Mr Johnson said there was a "ready supply" of lateral flow tests.
"If you can't get one online for any reason, then there are ample supplies in the shops. But what I think, if I may say so, what that also shows is that people are doing the sensible thing, and getting tests as well."
The UK Health Security Agency said "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."
And people eager to book their booster jab have also been advised to try "again later today or tomorrow" due to "extremely high demand".
Some have reported being in an online queue of over a thousand people.
In a tweet, the health service said: “The Covid vaccine booking service is currently facing extremely high demand so is operating a queuing system.
“For users aged 18-29, please be aware that booking opens on Wednesday 15 Dec.
“For all others experiencing waits, we would advise trying again later today or tomorrow.”
It comes on the first day that 30 to 39-year-olds in England can officially book the jab.
The service had already booked more than 140,000 vaccine appointments on Monday, NHS Digital said, with people waiting several minutes.
Mr Javid said "two [vaccine] doses aren't enough" to defend against Omicron, and people "need a third dose - a booster dose - to properly protect yourself".
"If we don't do this and we don't get more people boosted, then the impact on health care, including non-Covid health care if hospitals get full, will be much, much bigger," he added.
But Dr Angelique Coetzee, the South African physician who raised the global alarm on Omicron after it was first discovered in her country, said her experience is that the variant causes mild disease with very few increases in hospitalisations.
South Africa, which has had Omicron cases much longer than the UK, has not gone into lockdown over the variant, but Prime Minister Johnson said hospitalisations there are "doubling in a week".
The UK has now recorded at least 3,137 Omicron cases with at least 10 people in hospital.
Stephen Reicher, professor of psychology at the University of St Andrews, urged people to “think carefully” about their social contacts in the run-up to Christmas.
The member of government advisory body the Scientific Pandemic Insights Group on Behaviours (Spi-B) told ITV’s Good Morning Britain: “At the moment, we’re in a situation where the new variant in effect is coming at us like an express train.
“We’ve got to do something or else we’re in real danger of overwhelming our society and overwhelming the NHS.
“And there’s so many things you can do. The first thing, and the most obvious thing, is that if you reduce the number of social contacts you have you limit the spread of the infection."