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“I’d like to say thank you to all the brilliant NHS staff taking care of me and others in this difficult time,” he said in a tweet. “You are the best of Britain.”
Johnson was sent to a hospital Sunday as “a precautionary step, as the prime minister continues to have persistent symptoms of coronavirus 10 days after testing positive,” his office announced.
His symptoms are believed to include a high temperature, Sky News reports.
“Last night, on the advice of my doctor, I went into hospital for some routine tests as I’m still experiencing coronavirus symptoms,” Johnson tweeted Monday. “I’m in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe.”
Housing and Communities Secretary Robert Jenrick told the BBC that Johnson is awaiting the results of medical tests after spending the night at the undisclosed hospital.
“I’m sure this is very frustrating for him, for somebody like Boris who wants to be hands-(on) running the government from the front, but nonetheless he’s still very much in charge of the government," he said.
Jenrick did not rule out a more prolonged stay, but noted that Johnson had been working “phenomenally hard’’ in this “uniquely intense period."
The 55-year-old leader had been quarantined in his 10 Downing Street residence since being diagnosed with COVID-19 on March 26 — the first known head of government to fall ill with the virus.
He has continued to preside at daily meetings on the outbreak and has released several video messages during his isolation.
“This isn’t an emergency admission, and so I certainly expect that he will be back at Number 10 shortly," Jenrick said about Johnson's ongoing hospital stay.
As of Monday, the United Kingdom has 47,806 confirmed coronavirus cases, with 4,934 deaths, according to statistics from Johns Hopkins University.
Queen Elizabeth II addressed the U.K. on the crisis Sunday, saying that “together we are tackling this disease and I want to reassure you that if we remain united in resolute then we will overcome it.”
Fox News’ Frank Miles and The Associated Press contributed to this report.