British Prime Minister Boris Johnson was moved to the intensive care unit of a London hospital after his coronavirus symptoms worsened Monday, just a day after he was admitted for what were said to be routine tests.

Associated Press reports that Britain has no official post of deputy prime minister, but Foreign Secretary Dominic Raab has been designated to take over should Johnson become incapacitated.

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.

Downing Street said Johnson 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.

Hours earlier, Johnson tweeted that he was in good spirits and thanked the National Health Service for taking care of him and others with the disease.

“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. “I’m in good spirits and keeping in touch with my 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.

Johnson’s spokesman, James Slack, refused to say what kind of tests Johnson was undergoing. He insisted that “the PM remains in charge of the government.”

“He is receiving updates in hospital and is continuing to receive a (ministerial red) box” of files and briefing papers, Slack said.

The 55-year-old leader 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.

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.

“He’s in charge, but he’ll continue to take doctors’ advice on what to do next,” Raab said.

Politicians from all parties wished Johnson a quick recovery. Opposition Labour Party leader Keir Starmer tweeted: “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.