Covid-19 cases recorded in a single day have risen above 40,000 for the first time, as Public Health England (PHE) warned "hospitals are at their most vulnerable".
The Government said that, as of 9am on Monday, there had been a further 41,385 lab-confirmed cases of coronavirus in the UK, while a further 357 people have died within 28 days of testing positive.
Dr Yvonne Doyle, medical director at PHE, said: "This very high level of infection is of growing concern at a time when our hospitals are at their most vulnerable, with new admissions rising in many regions."
It comes as hospitals in the South face a rise in pressure as the number of coronavirus patients receiving treatment heads towards the April peak.
Saffron Cordery, deputy chief executive of NHS Providers, said: "We know that the rate of Covid-19 admissions is rising and some trusts are reporting up to three times the number of Covid patients than at the peak of the first wave.
"This means hospitals and also ambulance services in Tier 4 areas and beyond are incredibly busy, compounded by increasing staff absences due to illness and the need to self-isolate."
Dr Nick Scriven, immediate past president of the Society for Acute Medicine, called the trend "extremely worrying" and said "systems will again be stretched to the limit".
"It is not 'just the case' of using the Nightingale hospital as there are simply no staff for them to run as they were originally intended (mini intensive care units)," he said.
"They could play a role perhaps if used as rehabilitation units for those recovering but, again, where do we find the specialist staff – the NHS simply does not have the capacity to spare anyone."
Paramedics in the capital received almost 8,000 callouts on Boxing Day, which was described as one of London Ambulance Service's (LAS's) "busiest ever days", up more than 2,500 on the 5,217 received on the same day last year.
Medics are receiving support from other ambulance services in the South but remain very busy, with LAS tweeting on Monday: "Yesterday (December 27) we received more than 7,000 calls. We'll take 5,500 calls on a typical 'busy' day."
Yesterday we received more than 7,000 calls. We'll take 5,500 calls on a typical 'busy' day.
Please continue to help us help you – only call 999 in an emergency. For urgent medical advice when it's not an emergency, please go to NHS111 online first: https://t.co/z1JoP2fYFE.
— London Ambulance Service (@Ldn_Ambulance) December 28, 2020
Dr Katherine Henderson, the president of the Royal College of Emergency Medicine, described her experience of working in a hospital on Christmas Day as one of "wall-to-wall Covid".
She told BBC Breakfast: "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-Covid activities going.
"So we will stretch staff, the problem is at the moment we have a lot of staff sickness."
Professor Jackie Taylor, president of the Royal College of Physicians and Surgeons of Glasgow, told the programme London and the South East are now experiencing what Scotland went through in the autumn.
Meanwhile, an outbreak of further positive Covid-19 cases at Manchester City led to their Premier League trip to Everton being postponed just four hours before kick-off.
NEWS This evening's game at Everton has been postponed.
— Manchester City (@ManCity) December 28, 2020
The Government said a further 357 people have died within 28 days of testing positive for Covid-19 as of Monday, bringing the UK total to 71,109.
More than six million people in east and south-east England went into the highest level of restrictions on Saturday, which now affects 24 million people representing 43% of the population.
Lockdown measures are also in place across the other three home nations, after mainland Scotland entered Level 4 restrictions from Saturday for three weeks, and a similar stay-at-home order is also in place in Wales.
Northern Ireland has also entered a new six-week lockdown, and the first week measures are the toughest yet, with a form of curfew in operation from 8pm, shops closed from that time and all indoor and outdoor gatherings prohibited until 6am.
Calum Semple, professor of outbreak medicine at the University of Liverpool and a member of the Scientific Advisory Group for Emergencies (Sage), described the Oxford/AstraZeneca vaccine as a "game changer" if it is approved by the Medicines and Healthcare products Regulatory Agency (MHRA) in the coming days but warned "herd immunity" through vaccination would not be likely until the summer.
Meanwhile, Downing Street and Department for Education (DfE) officials were due to meet on Monday to discuss whether schools should be kept open if tougher measures are needed, although the DfE declined to comment on the outcome.
But Cabinet Office Minister Michael Gove said the Government hopes the staggered reopening of schools in England will go ahead in January as planned.
A DfE spokeswoman added: "It is right that we follow the path of the pandemic and keep our approach under constant review."