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Johnson's office said the 55-year-old prime minister has been conscious and did not need ventilation. He asked Foreign Secretary Dominic Raab to deputize for him, the spokesman said.
“Over the course of this afternoon, the condition of the prime minister has worsened and, on the advice of his medical team, he has been moved to the intensive care unit at the hospital,” the spokesman added. “The PM is receiving excellent care, and thanks all [National Health Service] staff for their hard work and dedication.”
The move came just a day after Johnson was admitted for what were said to be routine tests.
Downing St. said Johnson was in the ICU in case he needed ventilation later.
The prime minister’s spokesman said Johnson had spent a comfortable night and remained in charge of government despite being admitted to the hospital after COVID-19 symptoms of a cough and fever persisted.
Johnson had been quarantined in his home since being diagnosed with COVID-19 on March 26.
Britain has no official post of deputy prime minister, but Raab has been designated to take over should Johnson become incapacitated.
Speaking at the government’s daily coronavirus news briefing, Raab said Johnson was being “regularly updated,” but admitted he had not spoken to him since Saturday.
“He’s in charge, but he’ll continue to take doctors’ advice on what to do next,” Raab said.
Hours earlier, Johnson tweeted that he was in good spirits after spending the night in the hospital. The prime minister's spokesman said Johnson had spent a comfortable night and remained in charge of government despite being admitted after COVID-19 symptoms of a cough and fever persisted.
Johnson's tweet thanked the NHS for taking care of him and others in this difficult time.
"On the advice of my doctor, I went into hospital for some routine tests as I'm still experiencing coronavirus symptoms," Johnson wrote in the tweet. "I'm in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe."
Johnson's fiancée, charity adviser Carrie Symonds, said she experienced some coronavirus symptoms but it wasn't confirmed whether she had contracted the virus. Johnson and Symonds previously announced they're expecting a baby later this year.
President Trump said at the White House Sunday: "Before I begin, I want to express our nation's well wishes to Prime Minister Boris Johnson as he wages his own personal fight with the virus. All Americans are praying for him. He's a friend of mine. He's a great gentleman and a great leader. And he's as you know, he was brought to the hospital today. But I'm hopeful and sure that he's going to be fine. He's strong man, strong person."
Fox News' Benjamin Hall in London, Matt Leach, Gillian Turner and The Associated Press contributed to this report.