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He was admitted Sunday night due to a high temperature that doctors said required additional tests. Downing Street officials said he was hospitalized as a “precautionary step” and that he still remained in charge of the government.
“The PM is still at hospital,” a British government source told Reuters. “He spent the night in hospital.”
Johnson, 55, had reportedly been suffering persistent coronavirus symptoms 10 days after he tested positive.
“On the advice of his doctor, the prime minister has tonight been admitted to hospital for tests. This is a precautionary step, as the prime minister continues to have persistent symptoms of coronavirus 10 days after testing positive for the virus," a spokesman said on Sunday.
Since he tested positive, the Prime Minister had been isolating in Downing Street. On Friday, he announced on Twitter that he was "feeling better," although he still had one of the symptoms described as "minor" after completing seven days of isolation.
"I still have a temperature and so, in accordance with government advice, I must continue my self-isolation until that symptom itself goes," he added.
On March 27, he became the first leader of a major power to announce he had tested positive for COVID-19. At the time, he told the public he developed "mild symptoms" -- "a temperature and a persistent cough."
He's still been conducting government business remotely, as well as giving regular video messages to the public. Some of those messages have shown how the virus has physically impacted him.
As of Monday morning, the U.K. has over 48,000 confirmed coronavirus cases and at least 4,940 deaths, according to data from Johns Hopkins.
Fox News' Frank Miles and Adam Shaw contributed to this report