The British PM has been in intensive care since Monday after 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", according to Downing Street.
"The government's business will continue," Raab pledged, adding that Johnson was "in safe hands" at St Thomas' Hospital 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.
I send all my support to Boris Johnson, to his family and to the British people at this difficult moment. I wish him a speedy recovery at this testing time.
— Emmanuel Macron (@EmmanuelMacron) April 6, 2020
French President Emmanuel Macron posted a message of support for the British prime minister.
US President Donald Trump told a press briefing he had asked leading US drug companies "to contact London" and "see if we can be of help".
EU Commission President Ursula von der Leyen also wished Boris Johnson a "speedy and full recovery".
Johnson, who confirmed that he had tested positive for Covid-19 on 27 March, 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.
Another quick update from me on our campaign against #coronavirus.
— Boris Johnson #StayHomeSaveLives (@BorisJohnson) April 3, 2020
You are saving lives by staying at home, so I urge you to stick with it this weekend, even if we do have some fine weather.#StayHomeSaveLives pic.twitter.com/4GHmJhxXQ0
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.
He was given oxygen ealrier on Monday.
Without precedent
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.
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 criticised for initially refusing to follow other European countries in requiring people to stay home as the virus spread.
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.
Her Majesty The Queen addresses the UK and the Commonwealth in a special broadcast recorded at Windsor Castle. pic.twitter.com/HjO1uiV1Tm
— The Royal Family (@RoyalFamily) April 5, 2020
The 93 year old Queen Elisabeth II 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 has said the monarch was being kept informed of developments.
