Some hospitals in England are at risk of becoming Covid-only sites, with rising admissions for the virus forcing trusts to cut back on other services.
There are now 26,500 Covid patients in hospital, meaning nearly a third of all people in hospital have the virus.
In London, half of all patients being treated in hospital have Covid.
It has prompted many hospitals to cancel routine operations to make more space - and there are now signs this is starting to happen for cancer care too.
Doctors warned the impact on the NHS could be "catastrophic".
It comes after a surge in patients in recent weeks.
The number of Covid patients has risen by more than 50% since Christmas - and is forecast to continue doing so.
There are now more than 3,000 new admissions a day on average, three times the normal winter rate for all respiratory conditions.
Chris Hopson of NHS Providers, which represents hospital managers, said the rising number of Covid cases was "very worrying" and had reached a "critical point", which was significantly affecting other services.
He said that while the south east and east of England had borne the brunt of the rises, other parts of England were seeing rises too.
In the last week alone there had been enough new Covid patients to fill 10 hospitals, he added.
Dr Nick Scriven, of the Society for Acute Medicine, said medical units, respiratory wards and elderly care wards had effectively become intensive-care areas with very sick patients being given oxygen therapy.
He described the pressure on staff as "intense".
Care 'not as good as it should be'
Meanwhile, Prof Rupert Pearse, an intensive care consultant at the Royal London Hospital and member of the Intensive Care Society, said staffing levels in intensive care were being diluted to cope with one nurse to every three patients rather than the normal one-to-one.
"We are really very concerned now about the seriousness of the situation ... which is definitely worse than the first wave and proving much harder to deal with now as the resources we had in the first wave aren't available to us.
"So we're really struggling to provide the quality of patient care that we think patients deserve. And the impact of the pandemic is taking care away from other illnesses such as cancer and heart disease.
"In essence, the healthcare available to all of us is not as good as it should be right now."
Prof Pearse said that unless people took the lockdown seriously, the impact on healthcare across the country "could be catastrophic".
The surge in cases has prompted increasing amount of non-Covid care to be stopped.
This week, it emerged that potentially lifesaving cancer operations have been put on hold King's College Hospital in London because of the number of beds taken by Covid patients.
This included all "priority two" operations - those doctors judge need to be carried out within 28 days.
Prof Neil Mortensen, president of the Royal College of Surgeons of England, said he had heard from members that "hospitals across London are having to cancel cancer surgeries as a result of the huge number of Covid-19 patients being hospitalised".