Britain on Tuesday faced warnings of an impending hospital crisis due to staff shortages caused by a wave of Omicron infections, as the country returned to work after Christmas.

Daily case numbers chalked up multiple records in the run-up to New Year, with Prime Minister Boris Johnson promising action to plug staffing gaps in the worst-hit areas.

Hospital admissions have not hit anything like the peaks of previous waves of the pandemic and the number of people requiring ventilation has remained flat so far.

But the state-run National Health Service (NHS), which is regularly under strain during the winter months from seasonal respiratory infections, is struggling with staff forced to stay at home after testing positive.

Transport networks also battled absences, leaving commuters facing long waits on their return to work, while other public services such as household bin collections were hampered.

Minister for vaccines and public health Maggie Throup told Sky News it was unclear how many Britons were currently in self-isolation after the Christmas surge in the highly transmissible virus mutation.

"But what is good news, it doesn't seem to be resulting in severe diseases as some of the other variants did," she told Sky News.

Around 50,000 NHS staff were absent from work last week because they were ill or self-isolating, The Sunday Times has reported.

At least six hospital trusts have declared critical incidents, which mean crucial services may be under threat.

'Almost impossible'

Matthew Taylor, chief executive of the NHS Confederation which represents healthcare providers in England, Wales and Northern Ireland, said cases appeared to have levelled off in London.

But he told Times Radio that staff shortages and rising cases across the rest of the country were a concern.

"The unpredictability of staff absence means NHS leaders having to work around the clock just thinking about how they can deploy their resources best to deal with the most urgent and pressing needs," he added.

"Even using all their imagination and creativity, it is becoming almost impossible, which is why we see hospitals declaring critical incidents."

Anyone testing positive for the virus has to self-isolate for 10 days, or seven days if they produce a negative test.

There have been calls to ease those restrictions for health staff to ease what Johnson on Monday said was "considerable pressure" on hospitals.

Taylor said reducing the self-isolation period to five days should only be done if the data indicated it was "absolutely safe".

Pupils returning to school also faced the prospect of mass staff absences and merged classes.

Secondary school pupils have been advised to wear face masks in class as a "temporary" measure.

Britain has seen nearly 149,000 deaths since the start of the pandemic in early 2020 and Johnson has defended his decision not to increase restrictions over Christmas in England, unlike in other parts of the UK.

A review of restrictions is expected this week.