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A record 78,610 lab-confirmed Covid-19 cases have been reported in one day in the UK, the highest daily total since the pandemic began.
The previous record of 68,053 on January 8, 2021 is more than 10,000 less than the cases reported in the 24 hours to 9am on Wednesday, amid the "staggering" speed at which the Omicron Covid variant is spreading.
The strain is presenting "probably the most significant threat" of the pandemic so far, according to head of the UK Health Security Agency Dr Jenny Harries, and it could place the NHS in "serious peril".
It comes ahead of a Downing Street press conference set to be led by the PM at 5pm this evening - he will be joined by Chief Medical Officer Chris Whitty and Dr Nikki Kanani, a medical director at NHS England.
Top government adviser Dr Harries said the Omicron variant's main threat is it "runs the risk of evading our natural and/or vaccine immunity" due to its vast number of mutations.
Even if the strain causes "milder" disease, as has been reported, it "could have very significant impact on our health services" because of its extremely fast transmissibility, she said.
"We can foresee a very large wave of Omicron coming through," she said, "and our health services potentially being in serious peril".
Robert Peston has been told Covid cases could hit a record of 75,000 daily cases "today or tomorrow".
It comes ahead of a Cobra meeting between governments in Westminster, Wales, Scotland and Northern Ireland this afternoon.
Nicola Sturgeon issued fresh guidance on Tuesday for Scots to have a "small" Christmas Day celebration and urged them to only socialise with two other households before and after December 25.
Last week's meeting was held to discuss how nations of the UK could coordinate their response to Omicron and news of today's has led people to speculate about new restrictions being brought in.
But the transport secretary told ITV News there will be no more coronavirus restrictions will be enforced in England before Christmas, despite the risks presented by Omicron.
Grant Shapps said "that's it for now" when asked if the government could go any further than the Plan B measures that were approved by MPs on Tuesday night.
The senior minister said Parliament would be recalled and asked to approve any emergency measures if they became necessary, but "we're not planning any further changes, we've already said that people can enjoy their Christmas".
Face masks, self-isolation rules, Covid passports and mandatory vaccines for NHS staff have all become law in England, despite Boris Johnson suffering a huge and highly damaging rebellion from almost 100 of his backbench MPs.
The embattled prime minister has been in the trenches over coronavirus restrictions after allegations he broke his own Covid rules during last winter's lockdown, but he managed to get his proposals through after relying on Labour votes.
His contentious Plan B means large venues will require all customers to provide proof of their Covid status, either with an NHS vaccine pass or a negative test result, while health workers will need to double jabbed to remain in their jobs.
Will there be more Covid restrictions before Christmas?
Both Mr Shapps and Dr Harries urged people to get their booster jabs in order to protect themselves against the variant.
She said the chances of double-jabbed patients fighting off infection is "much reduced" by Omicron but "we do know that the booster dose will push [immunity] right back up".
Mr Shapps said getting a booster jab, along with regular testing, will mean people can "enjoy their Christmas and get together with family and friends".
Despite advice on lateral flow tests, people wanting home deliveries have been unable to do so for the past three days.
Government websites reported that rapid lateral flow tests were "not available right now", the third day of problems, as demand surged.
People trying to book walk-in PCR tests also encountered problems, being told they were "not available right now".
Dr Harries said demands for lateral flow tests "have been absolutely astounding".