Hospitals across the country are "very, very close" to being overwhelmed after surges in coronavirus patients.
A London A&E doctor has warned that the capital's hospitals will soon be unable to cope if coronavirus infection rates are not brought under control.
She said doctors and nurses are working extra shifts and long hours and described it as a "extremely serious situation".
Meanwhile, a senior doctor has described how staff at Wales' largest hospital are facing "massive" numbers of Covid patients and how his colleagues are "stretched to the limit".
And a chief nurse at Gloucestershire Hospitals NHS Foundation Trust said the health service is "truly in the eye of the storm".
London A&E doctor Dr Sonia Adesara said: "The hospitals are extremely busy - we have seen a massive rise in people coming in with Covid-19 over the past week and this is on top of an increase in the non-Covid cases we see at this time of year.
"Just like the first wave we are also suffering from staff shortages, staff are getting Covid-19 again and it is extremely difficult, the hospitals are very full."
Speaking to BBC Breakfast, she said: "We are working all-out in the NHS - doctors and nurses are having leave cancelled, they're doing extra shifts, they're working extra long hours but it's an extremely serious situation.
"The situation is untenable and I think we are very close to becoming overwhelmed."
Dr Adesara said ambulance queues are now commonplace as a shortage of beds means they are unable to offload their patients.
She said: "There's a difficult situation for having capacity for beds and we have to keep our Covid-19 and non-Covid patients separate.
"When you don't have any spare beds you get a backlog and that means you get a backlog of ambulances who can't off load their patients."
She added: "In April we were mainly just dealing with Covid-19 patients, and everything else we weren't really seeing in A&E, but at this time of year because of the cold weather as well we are seeing maybe half to three-quarters of patients coming to us with Covid-19 but we also have non-Covid patients to deal with as well."
One positive, she said, when compared to April is that medical staff are getting much, much better and more experienced at treating the virus.
"We are doing all that we can and we will continue doing all that we can to keep everyone safe and make sure everyone is cared for, but I do think if we continue with current rate of admissions we are very, very close to becoming overwhelmed," she added.
Her comments come as a senior nurse has said the health service is "truly in the eye of the storm" as Covid-19 infection rates continue to rise.
Professor Steve Hams, a chief nurse at Gloucestershire Hospitals NHS Foundation Trust, told BBC Breakfast: "We've had a 30% increase in the community transmission rate over the last week, we currently have 200 (Covid-19) patients in our hospital beds, 10 in critical care."
During the first spike in April, the trust had had 60 Covid-19 patients, Prof Hams said.
He added: "We have seen that exponential rise. We truly are in the eye of the storm at the moment."
"It has probably been one of the most challenging times of my 25-year nursing career but one that I am most proud of."
He added: "I think it is fair to say that I - like many of our colleagues across the country - am becoming increasingly exhausted."
Dr Andrew Lansdown, a consultant endocrinologist at the University Hospital of Wales in Cardiff, was speaking on BBC Radio Five Live Breakfast on Monday - the day after Cardiff and Vale Health Board made an urgent appeal for help in intensive care.
Dr Lansdown, whose ward has recently been turned into a Covid ward, said: "It's starting to sound a bit like a cliche but it really is true that these are unprecedented times, we really have felt stretched to the limit.
"In the past week or two things have become relentless, we're under extreme pressure and the numbers of patients coming through now with coronavirus are massive.
"My ward has recently turned into a coronavirus ward to make way for these patients and on the weekend [we had] to tweet for extra help because of the impact of that.
"It's not just the new variant, which seems to be spreading faster, you've also got the usual winter pressures at the hospital as well as staff themselves who are ill, self-isolating.
"The workforce really is a challenge, so we're very much feeling the pressure at the moment."
Two Kent hospital trusts have issued warnings over the number of patients seeking treatment for coronavirus over the Christmas period, Kent Live reports.
Darent Valley Hospital has issued a statement urging non emergency patients to seek other NHS services due to a 'high numbers' of Covid-19 patients in the hospital.
The Queen Elizabeth Hospital in Greenwich has also declared an internal incident due to a large amount of Covid patients being admitted.
