Mr. Johnson wrote Monday on Twitter from a hospital in London that he was “in good spirits.”
Mr. Johnson, 55, first experienced the symptoms of the virus on March 26, was tested that day and received the positive result around midnight, going into self-isolation in Downing Street, but chairing meetings by videolink.
He was originally expected to resume normal working at the end of last week, but on Friday Mr. Johnson’s aides said that he still had symptoms, especially a high temperature, and had not ended his self-isolation. In a video statement in which he explained the situation, he still looked unwell.
Several other key figures in the government have self-isolated after suffering symptoms of the coronavirus, including the health secretary, Matthew Hancock, who has now returned to work, and the chief medical officer, Chris Whitty. Mr. Johnson’s partner, Carrie Symonds, who is pregnant, disclosed on Saturday that she, too, is suffering symptoms.
Mr. Johnson was initially criticized for his slow response to the outbreak, but later moved to place Britain under a virtual lockdown, closing all nonessential shops, banning meetings of more than two people, and requiring people to stay in their homes, except for trips for food or medicine.
Before that his government had tried to take a different tack from the rest of Europe, which moved earlier to close businesses to try to halt the spread of the virus. Some officials, including Sir Patrick Vallance, England’s chief scientific adviser, had said that the government was looking “to build up some kind of herd immunity so more people are immune to this disease and we reduce the transmission.”
At one news conference in early March, Mr. Johnson 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.”
The announcement of Mr. Johnson’s hospitalization came hours after Queen Elizabeth II issued a rare televised address on Sunday, attempting to rally her fellow Britons to confront the pandemic with the resolve and self-discipline that have seen the nation through its greatest trials.