One in four NHS doctors are off work sick or in self-isolation, the head of the Royal College of Physicians has said.
A leading expert has also revealed there were signs of a slowdown in Covid-19 hospital admissions.
Professor Andrew Goddard, president of the RCP, said that about 25% of the doctor workforce is off, either with coronavirus or because a family member or housemate is ill.
"At the moment, we think it's more doctors self-isolating with family members, though there are some off sick themselves," he said.
"This is really impacting a lot in emergency departments and London is in a much worse position than elsewhere at the moment, but it will come to other places.
"Birmingham is also struggling."
Prof Goddard said hospital wards across England "are going from normal wards to Covid wards very quickly".
Asked about the pressure on intensive care units, Prof Goddard said: "Some hospitals are really at the limit.
"Within London it's very, very difficult at the moment, you can't underestimate how difficult it is."
He said it was unclear whether the 25% off work would be a "rolling number" or whether it could ease as testing of NHS staff increases and people come out of isolation.
"Of course the worry is we will lose more people to covid-related illness," he added.
It comes after the Royal College of Nursing said on Sunday that around one in five nurses had taken time off work to self-isolate.
Meanwhile, Professor Neil Ferguson from Imperial College London and author of a report which warned of mass deaths if the UK did not introduce strict controls, said there were signs the rate of hospital admissions was slowing.
It comes as Dominic Cummings, the Prime Minister's chief adviser, became the latest in Westminster to self-isolate after developing symptoms.
He joins Boris Johnson, Health Secretary Matt Hancock and England's chief medical officer Professor Chris Whitty, who are all in self-isolation due to Covid-19.
Prof Ferguson said social distancing measures brought in by the Government appeared to be having an effect on the numbers.
"In the UK we can see some early signs of slowing in some indicators - less so deaths because deaths are lagged by a long time from when measures come in force," he told the BBC Radio 4 Today programme.
"But if we look at the numbers of new hospital admissions, that does appear to be slowing down a bit now.
"It has not yet plateaued, so still the numbers can be increasing each day but the rate of that increase has slowed."
Prof Ferguson said the epidemic was spreading at different rates in different parts of the country, but across the UK perhaps 2% or 3% of the population had been infected.