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Johnson, who was isolating in Downing Street after testing positive last month, still had a high temperature and so his doctors felt he should go to hospital for tests. He remains in charge of the British government, his Downing Street office said.
"On the advice of his doctor, the prime minister has tonight been admitted to hospital for tests," Downing Street said.
"This is a precautionary step, as the prime minister continues to have persistent symptoms of coronavirus ten days after testing positive for the virus," the statement added.
News of his hospitalisation came only after an hour after Queen Elizabeth delivered a rallying call to the British public saying they would overcome the coronavirus outbreak if they stayed resolute.
Johnson, 55, on March 27 became the first leader of a major power to announce that he had tested positive. He went into isolation at an apartment in Downing Street and said on Friday he was staying there as he still had a high temperature.
"Although I'm feeling better and I've done my seven days of isolation, alas I still have one of the symptoms, a minor symptom, I still have a temperature," a weary-looking Johnson, sitting in a chair with his shirt open at the neck, said in a Twitter video message on Friday.
Downing Street underscored that this was not an emergency admission and that Johnson remains in charge of the government.
Sterling opened Monday trading at $1.2288 vs the dollar and fell to $1.2233 at 2014 GMT after the news and is now at $1.2230.
"Wishing the prime minister well and a speedy recovery," Keir Starmer, the newly elected leader of the opposition Labour Party, said.
CORONAVIRUS IN THE UK
Johnson has faced criticism in the United Kingdom for initially approving a much more modest response to the novel coronavirus outbreak than other major European leaders.
But he swiftly changed tack when projections showed a quarter of a million people could die in the United Kingdom.
He effectively shuttered the world's fifth-largest economy, advising people to stay at home and the elderly or infirm to isolate themselves for weeks.
But the virus penetrated the British government. Johnson and his health minister tested positive last month and his chief medical adviser also self isolated.
Carrie Symonds, Johnson's 32-year-old pregnant fiancée, said on Saturday that she had spent the past week in bed with symptoms of the novel coronavirus but after seven days of rest felt stronger and was on the mend.
"The prime minister thanks NHS staff for all of their incredible hard work and urges the public to continue to follow the government's advice to stay at home, protect the NHS and save lives," Downing Street said.
The United Kingdom's death toll from the coronavirus rose by 621 to 4,934 at 1600 GMT on April 4, the health ministry said on Sunday. As of 0800 GMT, a total of 195,524 people had been tested of which 47,806 tested positive.