Prime Minister Boris Johnson began a third day in intensive care on Wednesday battling the coronavirus, which has struck at the heart of the British government, infected more than 55,000 people across the country and killed nearly 6,200.

"I understand the prime minister is in a stable condition, he's comfortable and in good spirits," Edward Argar, a junior health minister, told Sky News. 

"He has in the past had some oxygen but he's not on ventilation."

The Times reported Johnson's persistently high temperature had fallen, while the Daily Telegraph said he was being cared for by one of Britain's leading lung doctors. 

Newspapers urged Britons to keep their stricken leader at the forefront of their minds, with the country in lockdown to try to stem the spread of COVID-19 in its third week.

"He stayed at work for you... now pray at home for him," The Sun tabloid splashed across its front page. "Boris 'will pull through'" said the Daily Express.

Deputising for Johnson, Foreign Secretary Dominic Raab called him "a fighter" and predicted "he'll be back, leading us through this crisis in short order".

Johnson, 55, is the most high-profile government leader to become infected with COVID-19 and messages of support flooded in from across Britain and the world.

He was admitted to intensive care on Monday evening after spending Sunday night in hospital following concerns he still had a cough and high temperature 10 days after being diagnosed with COVID-19.

His transfer to intensive care is unprecedented for a prime minister during a national emergency.

For many people, it brought home the seriousness of the disease that has so far seen 6,159 deaths in Britain, with a record 786 more reported in a daily update on Tuesday.

'Work goes on'

Despite the record daily death toll, there was more encouraging news with the number of new daily cases remaining at a roughly stable 3,643.

Raab chaired the daily coronavirus meeting in the prime minister's place on Tuesday.

He later insisted it was still too soon to say whether the stringent social distancing measures introduced on March 23 for an initial three-week period would be eased at all.

Downing Street later confirmed a review would take place next week as planned, after earlier dismissing suggestions of a power vacuum at the top of the British government.

"There is a clear plan... the government and the cabinet are working together to implement that plan," Johnson's spokesman said.

The country does not have a formal constitutional role of deputy prime minister, and experts said Raab would need the support of the rest of the cabinet to make any big decisions.

'Enormous shock'

Johnson announced on March 27 that he had coronavirus and went into self-isolation in a flat above his Downing Street office.

But on Monday evening he was moved to intensive care in London's St Thomas' hospital after his condition worsened.

The prime minister has received messages of support from around the world, with US President Donald Trump sending best wishes to his "very good friend" while Russian leader Vladimir Putin said Johnson's "energy, optimism and sense of humour" would see him through.

For some, Johnson's larger-than-life personality has made his hospitalisation all the more shocking.

His biographer Andrew Gimson said Johnson always made him feel upbeat, and "now here he is the stricken one".

"This is an enormous shock, completely unfamiliar territory for all those who know him," he told BBC radio.

Experts said it was not uncommon for coronavirus patients to move to intensive care, but said it showed Johnson's condition was serious.

"There is no doubt this turn of events means Boris Johnson is extremely sick," said Derek Hill, professor of medical imaging at University College London.

Still shaking hands

The British government was criticised for initially refusing to follow other European countries in requiring people to stay home as the virus spread rapidly across the globe.

Johnson himself said in early March that he was still shaking hands with people.

Two weeks ago, he ordered a nationwide lockdown, but parliament continued to sit for several days after and Westminster became a hotspot for the virus.

Health Secretary Matt Hancock and the heir to the throne, Prince Charles, have both been infected, although they have since recovered.

Johnson, who has been prime minister only since July last year, is not known to have any underlying health issues, although he has struggled with his weight.

Johnson's pregnant partner, Carrie Symonds, moved out of Downing Street after some staff fell ill.

But she said on Saturday she had just spent a week in bed with symptoms, although she has not been tested.