An estimated 1.7 million people had coronavirus across the UK last week, the highest number since comparable records began last year, new figures suggest.
The new Office for National Statistics (ONS) data covers people who tested positive for COVID-19 outside hospital or a care setting during the week ending 19 December.
In England there were 1.5 million people with the virus that week - the equivalent of one in 35 people.
The figure was highest in London, where most cases of the Omicron variant have been detected, at a rate of one in 20 people.
It was lowest in Scotland, where the figure was one in 65, followed by Wales with one in 45 and Northern Ireland at one in 40.
Officials still unsure of Omicron severity until it hits elderly
Commenting on the figures, UK Health Security Agency chief executive Dr Jenny Harries said the UK "hasn't seen rates like this" since the figures were first published in autumn 2020.
But she stressed that despite data published yesterday - that someone with Omicron is between 50% and 70% less likely to be admitted to hospital than a patient with Delta - officials are still unsure of how it will affect the elderly and clinically vulnerable.
"Most of the cases of Omicron in the UK so far have been in younger people," she told Sky News.
"We're only just starting to see cases tip into the over-60s and over-70s. This is a very, very mixed picture. So we need to be absolutely sure what the severity risk is."
National statistician Professor Ian Diamond said that although numbers of cases in the elderly are still small, "this is the first time in some months that we've recorded an increase in the over-70s".
He added that although London is the epicentre of the Omicron outbreak, there are regional variations, with the South West lowest for infections.
NHS shortages key to decision on post-Christmas restrictions
Speaking to the BBC's Radio 4 Today programme earlier, Dr Harries referred to the hospitalisation rate as a "glimmer of Christmas hope", but one that would not "downgrade the serious threat" around the new variant.
On Thursday Health Secretary Sajid Javid confirmed that no announcement on post-Christmas COVID restrictions would come before 27 December.
Dr Harries said that data on more vulnerable groups, alongside the effect the new variant is having on workforce shortages, particularly across health and social care, will determine what move the government makes on restrictions next week.
"The problem with Omicron is we've got lots and lots of cases, so even if the severity is potentially reduced, we still have to deal with these 119,000 daily cases," she said.
"We're concerned about people's health, and about transmission, but most of all that our health and care services can keep functioning.
"We had relatively high case rates before we had Omicron and the NHS was already facing winter pressures, so it's about what contribution Omicron is making."
Earlier today the head of the Royal College of Nursing (RCN) said that the NHS is facing a "very, very depleted workforce" and wants ministers to take action.
Pat Cullen told BBC Breakfast: "If that means tighter measures, that's for political leaders to decide based on the scientific evidence.
"They [NHS staff] would like to see political leaders making decisions that will support the health service and them to be able to do the job that they want to do and be able to care for their patients safely."
Chief of NHS Providers Chis Hopson also said that hospitals are "very, very busy" and the NHS is under "huge amounts of pressure".