It will be a “close run thing” whether the NHS capacity will be exceeded over the coming weeks because of the coronavirus outbreak, chief medical officer Chris Whitty has said.

At a press conference in 10 Downing Street, Prof Whitty said that there was not currently “enormous” pressure on critical care beds within the health service, despite a total of more than 8,000 patients testing positive for coronavirus across the UK.

But he said that he could not guarantee that bed spaces would not run out within the next three weeks.

The NHS has more than 4,000 critical care beds in normal times and efforts are under way to accommodate the expected surge in additional coronavirus patients by using private sector facilities and discharging patients able to go home.

The ExCel exhibition centre in east London is being converted into a field hospital which will eventually be able to take 4,000 patients during the outbreak.

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Prof Whitty said: “The NHS is increasing supply by a combination of pushing out in time things which can be postponed and increasing the critical care and particularly the ventilated bed capacity over the next weeks.”

But he added: “This is going to be a close run thing, we all know that.

“And anybody who looks around the world can see this is going to be difficult for every health system.”

Prof Whitty said that the lockdown announced by prime minister Boris Johnson on Monday, requiring people to stay at home as much as they can and avoid social contact, should help relieve pressure on bed spaces by reducing the rate of infection, while the NHS works rapidly to increase capacity.

“That is the way that we will narrow this down to the smallest possible gap over the next three weeks,” he said.

“And, you know, we do think that if everybody sticks to staying in your households unless absolutely essential, this gap will be probably manageable by the NHS.

“But we cannot guarantee that, and nobody who is sensible would wish to guarantee that, but we think that is what we are planning for and that is what we intend to happen.”

Current pressure on critical care beds is “not enormous … compared to a bad, or even normal, winter day in the NHS”, said Prof Whitty.

But he added: “We expect the demand for critical care beds to continue to rise over the next two weeks.