Prime Minister Boris Johnson was in intensive care on Monday after his coronavirus symptoms worsened, prompting concern for his health and well-wishes from political friends and foes in Britain and abroad, as the country battles to control the outbreak.
Foreign Secretary Dominic Raab -- Johnson's de facto deputy -- was immediately appointed to take over his duties "where necessary", Downing Street said.
"The government's business will continue," Raab pledged, adding that Johnson was "in safe hands" at St Thomas' Hospital, opposite parliament in central London.
"The focus of government will continue to be on making sure that the prime minister's... plans for making sure that we can defeat coronavirus... will be taken forward," he added.
Johnson, who confirmed that he had tested positive for COVID-19 on March 27, was last seen in a Twitter video on Friday, saying he still had a temperature and was staying in self-isolation.
He also tweeted from hospital at lunchtime on Monday that he was "in good spirits" and thanked the medical staff looking after him.
He and his office have repeatedly maintained that he only had mild symptoms and had been in constant contact with his senior ministers and advisers, chairing meetings by videolink.
But his admission to hospital on Sunday, less than an hour after Queen Elizabeth II called for a united front to defeat the virus, prompted concern his condition was more serious.
That was confirmed 24 hours later when Downing Street said his condition had "worsened and, on the advice of his medical team, he has been moved to the Intensive Care Unit at the hospital".
The government said he was moved as a precaution, in case he needed to use a ventilator.
Johnson is the most high-profile world leader with the disease that has spread rapidly across the globe, and his transfer to intensive care is unprecedented during a national emergency.
US President Donald Trump led world leaders in wishing Johnson a successful recovery. Other messages flooded in from the European Union and NATO to the World Health Organization.
Earlier on Monday, questions were raised about whether the Conservative leader could still run the country, and Raab said he had not spoken to Johnson in person since Saturday.
The British government was criticized for initially refusing to follow other European countries in requiring people to stay home as the virus spread.
Johnson himself said in early March that he was still shaking hands with people.
But two weeks ago he ordered a nationwide lockdown and Britain is now in the grip of a serious outbreak.
Over 50,000 cases and more than 5,000 deaths have been recorded so far, with a latest daily toll of 439.
Health Secretary Matt Hancock and the heir to the throne, Prince Charles, have both been infected with coronavirus, although they have since recovered.
The queen, 93, made a rare public address on Sunday night, evoking the spirit of World War II and urging Britons to stay united, just before Johnson went into hospital.
"We will be with our friends again, we will be with our families again, we will meet again," she said in a rare televised address watched by 23 million people.
Buckingham Palace said she was being kept informed of developments.
Housing minister Robert Jenrick told BBC television that Johnson had been working "phenomenally hard" during the crisis, adding that he would be finding it "very frustrating" to be ill.
Johnson is not known to have any underlying health issues, although he has struggled with his weight, but some questioned if he should have taken more time off.
Junior health minister Nadine Dorries, who also had coronavirus but has recovered, added: "Many with #COVID19 are felled by fatigue/temperature and use isolation to sleep and recover.
"Boris has risked his health and worked every day on our behalf to lead the battle against this vile virus."
James Gill, clinical lecturer at Warwick University medical school, said Johnson's condition "must underscore for everyone, across the world, how indiscriminate this virus is, ignoring class, character, wealth and position".
Trump said he was "hopeful and sure" Johnson would recover, calling the prime minister "a friend of mine" and a "great leader".
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.
Johnson's spokesman would not confirm a report in The Times newspaper reported that the prime minister had been given oxygen treatment.
"Doctors will be monitoring important vital signs such as oxygen saturations," said Rupert Beale, group leader at the cell biology of infection laboratory at the Francis Crick Institute.
He said they would also check Johnson's blood to "see what the immune response to the virus looks like, and to assess liver and kidney function", and may also perform an electrocardiogram to check the heart.
© Agence France-Presse