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In a video message posted to his Twitter account Friday, he explained he had developed "mild symptoms" over the last 24 hours -- "a temperature and a persistent cough."
"On the advice of the chief medical officer I have taken a test. That has come out positive, so, I am working from home," Johnson said. "I’m self-isolating. And that’s entirely the right thing to do."
"But, be in no doubt that I can continue thanks to the wizardry of modern technology to communicate with all my top team to lead the national fight back against coronavirus," he added.
The White House says President Trump spoke with Johnson today, where he "thanked the Prime Minister for his close friendship and wished him a speedy recovery."
Johnson's diagnosis comes just a couple of days after Prince Charles revealed a positive test. U.K. Health Secretary Matt Hancock announced Friday that he has it as well -- and like Johnson, is now working from home.
"I'll be self-isolating here until next Thursday," Hancock said.
The United Kingdom, like many other countries around the world fighting the coronavirus, has rolled out a series of orders for non-essential businesses to close and residents to remain indoors unless otherwise necessary.
Downing Street told the Sun that several staffers will now have to self-isolate too, although Rishi Sunak, the chancellor (effectively the treasury secretary) will not because he has shown no symptoms.
Johnson, in his video, praised the medical professionals, police, social care workers, teachers and the "600,000 people" he says have volunteered in a "great national effort" to contain the spread of COVID-19.
Thousands of citizens around the U.K. took to their windows, balconies and doorsteps Thursday night to applaud its National Health Services for their efforts in fighting the outbreak. Johnson, who was among the participants, described the event as "very moving."
“I want to thank everybody who is working to keep our country going through this epidemic. And we will get through it," he said Friday. "And the way we are going to get through it is, of course, by applying the measures that you have heard so much about. The more effectively we all comply with those measures, the faster our country will come through this epidemic and the faster we will bounce back.”
“Stay at home. Protect the NHS. And save lives," he concluded.
Jeremy Corbyn, the leader of the opposition Labour Party, says he wishes Johnson a "speedy recovery and hope his family are safe and healthy."