“He is comfortable, he’s stable, he’s in good spirits; while he’s had oxygen, he hasn’t been on a ventilator,” Edward Argar, a Junior Health Minister, said on Wednesday.
Johnson, who tested positive nearly two weeks ago, was admitted to St Thomas’ hospital on Sunday evening with a persistent high temperature and cough, but his condition deteriorated.
He was rushed into an intensive care unit.
The 55-year-old British leader has received oxygen support but was not put on a ventilator.
His designated deputy, Foreign Secretary Dominic Raab, said he would soon be back at the helm as the world faces one of its gravest public health crisis in a century.
As Mr Johnson battled the novel coronavirus in hospital, the United Kingdom was entering what scientists said was the deadliest phase of the outbreak and grappling with the question of when to lift the lockdown.
Inside the government, ministers were debating how long the world’s fifth-largest economy could afford to be shut down, and the long-term implications of one of the most stringent set of emergency controls in peacetime history.
The United Kingdom’s total hospital deaths from COVID-19 rose by a record 786 to 6,159 as of 1600 GMT on April 6, the latest publicly available death toll, though just 213,181 people out of the population of around 68 million have been tested.
Britain was in no position to lift the shutdown as the peak of the outbreak was still over a week away, London Mayor Sadiq Khan said.
“We are nowhere near lifting the lockdown,” Mr Khan said.
Mr Johnson was breathing without any assistance and had not required respiratory support, said Raab, who said the prime minister, whom he described as “a fighter”, remained in charge.
There are few precedents in British history of a prime minister being incapacitated at a time of major crisis, though Winston Churchill suffered a stroke while in office in 1953.
Also Tony Blair twice underwent heart treatment in the 2000s.
Mr Johnson has delegated some authority to Mr Raab, who was appointed foreign minister less than a year ago, though any major decisions – such as when to lift the lockdown – would in effect need the blessing of Mr Johnson’s cabinet.
Mr Raab said ministers had “very clear directions, very clear instructions” from Johnson but it was not clear what would happen if crucial decisions needed to be made which strayed from the approved plan.
Michael Heseltine, who served as deputy prime minister to John Major in the 1990s, told the Telegraph Mr Raab’s position needed to be clarified.
Former Foreign Secretary Malcolm Rifkind said most major decisions over the coronavirus strategy had been taken with the important exception of whether or not to ease the lockdown, a call that will need to be made in the next week or soon after.
“That is not just a medical judgement. It has to be a balance between the medical considerations and the consequences of leaving the whole economy shut down,” Mr Rifkind told BBC TV.
While such a decision would be made by cabinet even if Johnson were not unwell, he said Britain’s prime minister had authority and sway as the “primus inter pares” – Latin for “first among equals.
“He very often can steer the direction in a particular way. Dominic Raab doesn’t have the authority nor would he claim it,” Mr Rifkind said.