British Prime Minister Boris Johnson was battling the coronavirus in intensive care on Tuesday, raising serious concerns about his health and the government's response to the still-escalating outbreak.

"The prime minister has been stable overnight and remains in good spirits," his official spokesman told reporters at a daily briefing.

He added the 55-year-old Conservative premier was receiving "standard oxygen treatment and breathing without any other assistance" and had not required a ventilator.

Johnson is the most high-profile leader to become infected and messages of support flooded in from across Britain and the world. He was admitted to intensive care on Monday evening and asked Foreign Secretary Dominic Raab to deputise for him "where necessary", effectively putting him in charge.

Johnson had already spent Sunday night at a London hospital after being taken in for tests 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 killed 5,373 people in Britain. "It's pretty shocking, and also it just shows that it can affect anyone," said Mark Gillis, a Londoner who works for a media agency.

'Work goes on'

In a round of broadcast interviews, senior minister Michael Gove insisted the "work of government goes on". He later said he was now staying at home after a family member displayed mild coronavirus symptoms. "I have not displayed any symptoms and am continuing to work as normal," Gove added on Twitter.

Raab chaired the daily coronavirus meeting in the prime minister's place on Tuesday, and is now officially in charge. "The focus of the discussion was coordination of the government's coronavirus effort," the prime minister's spokesman said. "There is a clear plan... the government and the cabinet are working together to implement that plan," he said when asked if there was a power vacuum in Britain.

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. The most pressing issue is a review expected next week on whether to continue the nationwide lockdown introduced on March 23 to try to stem the spread of COVID-19.

London stocks jumped three percent at the opening on Tuesday despite Johnson's condition as investors anticipated an easing in the lockdown.

'Enormous shock'

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

Officials initially said he was only suffering mild symptoms, and the prime minister posted regular videos online giving updates on the government's work. His colleagues continued to insist on Monday that he was in charge.

But in the evening he was moved to intensive care in St Thomas' hospital after his condition worsened. US President Donald Trump sent his "best wishes to a very good friend of mine", while leaders in Europe, India, Canada and Australia urged his swift recovery.

Russian President Vladimir Putin said Johnson's "energy, optimism and sense of humour" would help him get better. For some, the prime minister'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 (UCL).

Slow to act?

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.

Queen Elizabeth II, 93, made a rare public address on Sunday, evoking the spirit of World War II and urging Britons to stay united in a broadcast watched by 23 million people. 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.