LONDON — Britain’s prime minister, Boris Johnson, who was hospitalized with persistent coronavirus-related symptoms, was moved to intensive care after his condition worsened on Monday, his office said.
Mr. Johnson had asked the foreign secretary, Dominic Raab, to deputize for him “where necessary,” 10 Downing Street added in a statement.
“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 statement said.
Mr. Johnson’s aides said that he remained conscious but had been moved to the intensive care unit as a precaution, in case he requires a ventilator to aid his recovery.
It is a dizzying turn of events for a government that, earlier this year, was brimming with confidence after a landslide election victory for the ruling Conservative Party in December.
The prime minister was admitted to St Thomas’ Hospital in central London on Sunday night, suffering from persistent symptoms of the coronavirus, including a high temperature and a cough.
Downing Street said that the decision to move him to intensive care was made by his medical team after his condition worsened Monday afternoon and that he was transferred around 7 p.m.
Before he was moved, the prime minister asked Mr. Raab to deputize for him where needed.
Earlier, Mr. Raab told a media conference that Mr. Johnson was in good spirits after a comfortable night in the hospital, a short distance from Downing Street. Mr. Raab said that Mr. Johnson remained “in charge” of the government and was working from his hospital bed.
But Mr. Raab, who chaired a daily government coronavirus meeting on Monday morning, admitted that he had not spoken to the prime minister since Saturday. The lack of details on the prime minister’s condition or any sign of ongoing communication between Mr. Johnson and Mr. Raab had caused some unease in Britain.
Earlier Monday, Mr. Johnson wrote from the hospital on Twitter that he had undergone “some routine tests” because he was still experiencing coronavirus symptoms. He also thanked health workers for taking care of him.
In a Twitter post, Andrew Neil, one of the country’s most prominent interviewers, asked why Mr. Johnson had not spoken to his stand-in, Mr. Raab, since Saturday. “Something not right here,” Mr. Neil added.
In Britain, patients who experience symptoms of the virus are being encouraged to stay at home if possible, a policy that suggested Mr. Johnson’s condition at least crossed some threshold of seriousness to warrant hospital admission at a time when the health service is stretched.
“This is terrible news. I know the thoughts and prayers of everyone across the House are with the Prime Minister and his family right now,” said Lindsay Hoyle, Speaker of the House of Commons, in a statement after the announcement “we all wish him a speedy recovery.”
The prime minister first experienced the symptoms of the virus on March 26, was tested that day and received a positive result around midnight, going into self-isolation in Downing Street, but chairing meetings by videolink.
Mr. Johnson was expected to have emerged from isolation and have resumed normal working duties by the end of last week. But on Friday his aides said that he had not shaken off symptoms, especially a high temperature, and had not ended his self-isolation. In a video statement in which he explained the situation, Mr. Johnson still looked unwell.
Even people with mild infections can have pneumonia, which is detected with CT scans. Sometimes the illness turns more severe in the second week — the stage where Mr. Johnson is now — and repeated CT scans may show worsening pneumonia. Blood tests may detect signs of inflammation, a sign that the immune system may be overreacting to the infection.
Until just weeks ago, Mr. Johnson had taken a more lax approach to the virus than had other many European leaders. At a news conference in early March, he described visiting a hospital where he said there were coronavirus patients.
“I shook hands with everybody, you’ll be pleased to know,” he said, “and I continue to shake hands.”
Later, the prime minister hesitated to force pubs and restaurants to close, even as he asked people to stop packing them, and he left schools open after France and Germany closed theirs.
Late on Sunday, shortly after Queen Elizabeth II made a televised address, the announcement came from Downing Street that Mr. Johnson had been hospitalized.
Aged 55, the prime minister is not thought to have any pre-existing medical conditions though he has admitted to struggling with his weight. In 2018 he wrote in an article that he had cut out some calorific food because he had reached about 230 pounds.
Several other key figures in the government have self-isolated after suffering symptoms of the coronavirus, including the health secretary, Matthew Hancock, and the chief medical officer, Chris Whitty, both of whom have now returned to work. Mr. Johnson’s partner, Carrie Symonds, who is pregnant, disclosed on Saturday that she, too, had experienced symptoms.
At Monday’s media conference, Mr. Raab said that it was too early to discuss an exit strategy from Britain’s lockdown, arguing that the focus should remain on social distancing measures designed to curb the spread of the virus.
According to statistics released on Monday more than 51,000 people had tested positive for the virus in Britain and 5,373 people had died, though there was some cause for optimism from figures that showed the rate of hospital admissions slowing.