Almost three million people across the UK tested positive for COVID in late December, the highest number since July.
The rise in infections comes as the UK faces its worst flu season for a decade and lengthy queues outside A&Es caused by ambulance handover delays.
One in 20 people in England are likely to have had COVID over the Christmas period, the Office for National Statistics said, with one in 18 infected in Wales and one in 25 testing positive in Scotland.
In Northern Ireland, one in 16 are estimated to have had COVID during the festive season, a nine-month high.
The COVID death rate remains low due to widespread immunity and a high level of vaccinations, although excess deaths in the run-up to Christmas were a fifth more than the five-year average.
No new variant is thought to have fuelled the uptick in infections, with experts pointing to increased indoor social mixing over Christmas.
Omicron subvariant XBB.1.5 is responsible for just one in 200 COVID infections in the UK, although it has become dominant in parts of the US.
ONS head of health surveillance Michelle Bowen said the spike in positive tests is largely visible in primary school-age children and people over 50.
She said: “Across English regions, infections have increased in the North East, Yorkshire and the Humber, the East Midlands, the East of England, the South East and the South West.
“Cases have also increased in those aged two to school Year 6, and those aged 50 years and over.”
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Cases peaked last winter at 4.3 million during the height of Omicron’s spread.
The number of infections then hit a record 4.9 million on 1 April last year.
Jonathan Ball, molecular virology professor at Nottingham University, said there is “no evidence” that XBB.1.5 is more dangerous than previous variants.
“It might be able to escape antibodies, but that’s not the only immunity we have,” he said.
“Our immune system is used to adapting to viruses.
“We’d better get used to the emergence of new variants, at least for the foreseeable future.
“Yes, they will lead to new waves of infection, but vaccination is still proving to be a very effective weapon to protect the most vulnerable from serious disease.”
The number of patients in general hospital beds in England shot up to 5,105 last week, up almost half on the previous week, NHS data showed.
Hospital admissions for flu were running at a decade-high in the days before Christmas, although it has since fallen.
Ambulance handover delays have also caused record wait times outside A&E.
In the run-up to Christmas, more than a quarter of patients waited in ambulances outside hospitals more than an hour and four in ten waited at least half an hour.
