Hospitals in the south of England say they have seen a "real rise in pressure" as the number of coronavirus patients needing treatment increases.
Saturday was described as one of London Ambulance Service's busiest in history amid the rapid spread of a new variant, the BBC reports.
The service and at least two others have urged people to call 999 only if there is a serious emergency.
Hospitals in Wales and Scotland are also reporting services being stretched by the impact of coronavirus.
The president of the Royal College of Emergency Medicine said she saw "wall to wall Covid" when she worked at one London hospital on Christmas Day.
Dr Katherine Henderson told BBC Breakfast there was a "great deal of difficulty" getting patients into wards.
She added: "The chances are that we will cope, but we cope at a cost - the cost is not doing what we had hoped, which is being able to keep non-coronavirus activities going.
"It is always challenging in winter, nobody would say that it wasn't, but at the moment the level of patient need is incredibly high."
The chief executive of Homerton hospital in east London urged people to comply with the tier four restrictions in force.
In a tweet, Tracey Fletcher wrote: "We are extremely busy... our nurses and doctors are incredibly stretched caring for very sick patients. No-one should underestimate the impact the infection can have."
Public Health Wales' Dr Giri Shankar said the system was under enormous and unprecedented pressure, with large numbers of patients - and staff sickness - creating an "incredibly challenging" situation.
And doctors have warned Scotland's health system is "severely stretched".
Prof Jackie Taylor, president of the Royal College of Physicians and Surgeons of Glasgow, said: "We are still seeing significant amounts of patients with Covid and large amounts of standard respiratory infections and other emergency that we see at this time of year that mount up to our standard winter pressures."
On Sunday, some 30,501 infections and 316 deaths were recorded in the UK.
But the true scale will be higher, as Scotland is not releasing data around deaths between 24 and 28 December, while Northern Ireland is not providing either case or death data.
There were 21,286 people in hospital with coronavirus across the UK on December 22 - the last day for which data is available - according to official government figures.
Cabinet Office minister Michael Gove said the government was pursuing the "right strategy - but the important thing to do is take into account the emerging scientific and medical evidence".
Some 24 million people in the south-east and east of England - about 43% of the population - are now living under the strictest tier four rules and a review on 30 December could see more areas placed under the restrictions.
Gove said: "I can't pre-empt that because it obviously has to be a judgement based on the medical situation. But... the NHS is under pressure and these are difficult months ahead."
Mainland Scotland entered level four restrictions from Saturday for three weeks, and similar rules are in place in Wales. Northern Ireland has also entered a new six-week lockdown.-Photo: PA Media
