Britain reported 88,376 new infections, the highest since the start of the pandemic and up around 10,000 since the previous record set on Wednesday.

The surge in cases was piling pressure on a health service struggling with staff sickness, England's Chief Medical Officer Chris Whitty said on Thursday.

Omicron is so transmissible that even if it proves to be milder than other variants, it could still cause a surge in hospital admissions, Whitty told lawmakers.

The record for the number of people admitted to hospital with COVID-19 is 4,583 set in January.

"It is possible, because this is going to be very concentrated over a short period of time, even if it's milder, you could end up with a higher number than that going into hospital on a single day," he said.

However, he said vaccinations could cut the numbers admitted to intensive care and shorten the time spent in hospital. On Thursday there were 849 admissions.

Susan Hopkins, the chief medical adviser at the UK Health Security Agency, said there were 15 proven cases of Omicron in hospitals, but that the number was likely to be much higher.

Although new cases were at a record high according to official data, Britain did not have mass testing capacity in March 2020 when the pandemic first hit the country, and so the scale of infections at that point is unknown.

A senior emergency doctor said hospitals, particularly in London, were struggling to maintain staffing levels due to the number who are having to isolate with COVID-19.

"Even if we are not seeing a big rise in hospitalisations yet, we are already seeing the effect on not having the staff to run shifts properly and safely," Katherine Henderson, an emergency consultant in London and president of the Royal College of Emergency Medicine, told BBC Radio.

"So we are worried about patient harm coming about because we just don't have the staff."

The education minister also warned of problems with staff shortages, and said his department would work with ex teachers who wanted to return to the profession to help.

Britain is betting that vaccine boosters will prevent serious illness from Omicron.

The government has also advised people to work from home, mandated mask wearing in public places and has introduced COVID-19 passes to enter some venues and events in England, but has stopped short of previous lockdown measures.

"If it looked as if the vaccines were less effective than we were expecting, that for example would be a material change to how ministers viewed the risks going forward," Whitty said.