There has been almost a 50% rise in just a few days in the number of people being treated for coronavirus in England's hospitals, according to new figures.
Sir Simon Stevens, chief executive of NHS England, said on Friday that more than 6,200 patients were in hospital with Covid-19.
But on Monday, he said this figure had jumped to more than 9,000.
England's chief scientific adviser Sir Patrick Vallance said the NHS was seeing around an additional 1,000 patients a day and described this daily rise as "stable".
He told reporters at the daily press briefing: "I do expect that number to continue. I expect people coming every day to be about that, it may go up a little bit.
"And in two or three weeks you would expect that to stabilise and to start to go down a bit."
It comes as 1,408 people are now confirmed to have died in UK hospitals after testing positive for Covid-19, as of 5pm on Sunday.
This is up 180 from 1,228 the day before.
Earlier the head of the Royal College of Physicians (RCP) said around one in four NHS doctors are off work sick or in isolation.
Public Health England (PHE) also announced that almost 11,000 coronavirus tests a day can now be carried out.
The Government had set a target of carrying out 10,000 tests a day by Sunday but PHE figures show 9,114 had been carried out as of 9am on Saturday and 8,278 had been carried out by the same time on Sunday.
PHE said fluctuation in the number of tests reported each day is to be expected but testing numbers are increasing overall.
The Office for National Statistics (ONS) will this week release figures on deaths involving Covid-19 in the wider community, such as care homes.
The ONS will look at deaths where Covid-19 was mentioned anywhere on the death certificate, including in combination with other health conditions.
These figures are likely to offer detail on how many community deaths involve coronavirus, rather than just looking at deaths in hospitals.
Sir Patrick said these ONS figures would provide "extra numbers" but they would not be "large."
Speaking on a visit to the new NHS Nightingale Hospital on the ExCeL site in east London, Sir Simon said that the number of patients will increase, but extra capacity is being made available.
He said: "Today, there are over 9,000 positive coronavirus patients in hospitals across England and we know that number is only going to increase.
"That's why what you see here is a mass mobilisation, taking place right across the country, but also at these new Nightingale hospitals.
"We have got available intensive care and available hospital beds but we are also bringing online additional capacity such as these Nightingale hospitals as we need them."
Earlier, Professor Andrew Goddard, president of the RCP, told the PA news agency that about a quarter 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 told PA: "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 (RCN) 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.
While the numbers are still going up, the rate of growth is slowing, he suggested.
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.
Based on the estimated UK population of 66 million, this would mean between 1.3 and two million people have or have had the illness.
– The Government and commercial airlines have formed a new arrangement to rescue tens of thousands of Britons stranded abroad by coronavirus, Foreign Secretary Dominic Raab said
– The owner of British Airways has extended by a year a credit facility which allows it to borrow from its lenders, as easyJet grounded its entire fleet and Carluccio's entered administration
– The Prince of Wales, who tested positive for Covid-19, is now out of self-isolation at his Birkhall home in Scotland
– Metropolitan Police Commissioner Cressida Dick has told officers that new powers to enforce lockdown rules should only be used as a last resort
On testing, Mr Hancock and Cabinet Office Minister Michael Gove both declared on Sunday morning that the 10,000 figure had been reached.
Earlier on Monday, University College London (UCL) announced that a breathing aid that can help keep Covid-19 patients out of intensive care has been developed by mechanical engineers, medics and the Mercedes Formula One team.
The device, known as continuous positive airway pressure (CPAP), has been used extensively in hospitals in Italy and China to help coronavirus patients and bridges the gap between an oxygen mask and the need for full ventilation.
Downing Street said the NHS had been given the go-ahead to order as many of the machines as it needs after trials were successful.