Professor Andrew Goddard said the level of illness and self-isolation was already seriously affecting emergency departments, particularly in London.
“Some hospitals are really at the limit,” he added. “Within London it’s very, very difficult at the moment, you can’t underestimate how difficult it is.
“It will come to other places. Birmingham is also struggling.”
He said it was still unclear whether the 25 per cent off work would increase or ease off as testing of NHS staff allows people to come out of isolation.
“At the moment, we think it’s more doctors self-isolating with family members, though there are some off sick themselves,” Professor Goddard added.
“Of course the worry is we will lose more people to Covid-related illness.”
He said hospital wards across England “are going from normal wards to Covid wards very quickly” to ease the pressure on intensive care units.
On Sunday the Royal College of Nursing (RCN) said around one in five nurses had taken time off work to self-isolate.
However there are “early signs” the rate of hospital admissions is slowing, according to the author of a report which warned of mass deaths if the UK did not introduce strict controls.
Professor Neil Ferguson from Imperial College London told BBC Radio 4 said: “It has not yet plateaued, so the numbers can still be increasing each day, but the rate of that increase has slowed.”
He said this evidence of slowing applied less to the mortality rate “because deaths are lagged by a long time from when measures come in force”.
“But if we look at the numbers of new hospital admissions, that does appear to be slowing down a bit now.
Prof Ferguson said two or three per cent of the population – up to two million people – had been affected the epidemic, although it was spreading at different rates in different parts of the country.
Additional reporting by agencies
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