Johnson was admitted to St. Thomas’ Hospital late Sunday, 10 days after he was diagnosed with COVID-19, the first major world leader to be confirmed to have the virus.
The 55-year-old Conservative was conscious and did not require ventilation, but he was moved into intensive care in case he needs it later, his office said in a statement.
Britain has no official post of deputy prime minister, but Foreign Secretary Dominic Raab has been designated to take over should Johnson become incapacitated.
A grave-looking Raab said Johnson was “in safe hands” at the hospital, which is treating many virus patients.
“The government's business will continue,” Raab said. He said Johnson had asked him "to deputize for him where needed in driving forward the government's plans to defeat coronavirus.”
“On the advice of my doctor, I went into hospital for some routine tests as I’m still experiencing coronavirus symptoms," Johnson said in the tweet. He said he was keeping in touch with his team “as we work together to fight this virus and keep everyone safe.”
The prime minister's spokesman said earlier Monday that Johnson had spent a comfortable night and remained in charge of government after being admitted to the hospital because his cough and fever persisted.
Speaking earlier at the government’s daily coronavirus press briefing, Raab said Johnson was being “regularly updated,” but said he had not spoken to him since Saturday.
Johnson had been quarantined in his Downing Street residence since being diagnosed with COVID-19 on March 26. He continued to preside at daily meetings on the outbreak until Sunday. He released several video messages during his 10 days in isolation urging Britons to stay home and observe social distancing measures to help slow the spread of the virus.
News that Johnson had been transferred to intensive care drew an outpouring of support from around the world.
“He’s been a really good friend," Trump said during a White House press briefing. "He’s been really something very special — strong, resolute, doesn’t quit, doesn’t give up.”
Trump said he asked two “leading companies” to contact officials in London about therapeutics that could be of help. He did not identify the companies, but said “we have contacted all of Boris’s doctors, and we’ll see what’s going to take place, but they are ready to go.”
French President Emmanuel Macron said in a tweet that he was sending his support to Johnson, his family and “the British people at this difficult time. I wish him well.”
Former Prime Minister David Cameron who went to school with Johnson, said in a tweet that he was thinking of Johnson and his family and that he was “in great hands, and we all want you safe, well and back in @10DowningStreet.”
The new leader of the main opposition Labour Party, Keir Starmer, said on Twitter: “Terribly sad news. All the country’s thoughts are with the Prime Minister and his family during this incredibly difficult time.”
Johnson's fiancee, Carrie Symonds, who is pregnant, is herself recovering from coronavirus symptoms.
Johnson was admitted to the hospital as a message to the nation from Queen Elizabeth II was being broadcast Sunday evening. The 93-year-old monarch urged the public to show resolve and follow advice to stay inside.
Buckingham Palace said the queen was being kept informed about Johnson's condition.
Concerns had been growing about Johnson’s welfare ever since he posted a message Friday in which he appeared red-eyed and flushed, saying that he was feeling better, though was still feverish.
Johnson has continued to work throughout his illness, to the concern of some of his colleagues. With the U.K. still approaching the peak of the coronavirus outbreak, both he and his government are under intense pressure.
The virus causes mild to moderate symptoms in most people, but for some, especially older adults and the infirm, it can cause pneumonia and lead to death.
The government said Monday that 51,608 people had been confirmed to have the coronavirus in Britain, 5,373 of whom have died.
Britain’s unwritten constitution does not have a clear rule for what happens if a prime minister becomes incapacitated or dies. Seven prime ministers have died in office, but the most recent was in 1865.
Johnson delegating Raab to fill in for him clarifies things for now, but it does not mean Raab would automatically take over permanently should a new leader be needed.
Jill Rutter of political think-tank U.K. in a Changing Europe, said Johnson’s illness should spur rival Cabinet ministers to unite and show that "they don’t require him there to actually be a functioning government.”
Derek Hill, a professor of medical imaging science at University College London, said it seemed that the prime minister needs help breathing, although he had no particular information on Johnson’s case.
There are multiple ways for doctors to support a patient's breathing, he said, including having oxygen on demand or being connected to a ventilator.
“It is quite common for people with COVID-19 to decline quite rapidly, and they may need to progress to invasive ventilators,’’ Hill said in statement.
Dr. James Gill, a family doctor and lecturer at Warwick Medical School, said Johnson's condition “must underscore for everyone, across the world, how indiscriminate this virus is, ignoring, class, character, wealth and position."
“For some people, the coronavirus outbreak just became real," he said.
Pan Pylas in London contributed to this story.