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U.K. Prime Minister Boris Johnson has tested negative for the coronavirus, his spokesman, James Slack, said Monday, one day after he was released from the hospital.

He will “not immediately returning to work,” Slack said Monday, adding that Johnson had spoken to Foreign Secretary Dominic Raab, who has been deputizing for the prime minister during his illness, over the weekend.

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Johnson, 55, was discharged from St. Thomas’ Hospital in London on Sunday and then went to Chequers, the prime minister’s country residence, around 40 miles northwest of the capital.

In a video message posted to Twitter after his release, the prime minister said: "It is hard to find the words to express my debt to the NHS for saving my life.”

"I thank you because so many millions and millions of people across this country have been doing the right thing, millions going through the hardship of self-isolation faithfully, patiently, and with thought and care for others as well as themselves," he said.

UK PRIME MINISTER BORIS JOHNSON RELEASED FROM HOSPITAL AMID CORONAVIRUS FIGHT

Johnson was admitted to St. Thomas’ on April 5 after his condition worsened and he was transferred the following day to its intensive care unit, where he received oxygen but was not put on a ventilator. He spent three nights there before moving back to a regular hospital ward.

On Sunday, Johnson's pregnant fiancee, Carrie Symonds, also expressed thanks for the "magnificent" NHS and staff at the hospital. She was also sickened with the virus.

"There were times last week that were very dark indeed. My heart goes out to all those in similar situations, worried sick about their loved ones," she tweeted.

Johnson was diagnosed over two weeks ago, becoming the first world leader confirmed to have the illness. His coronavirus symptoms at first were said to have been mild, including a cough and a fever, and he was working from home during the first few days before he was hospitalized.

March 17, 2020: Britain's Chancellor Rishi Sunak, left, and Prime Minister Boris Johnson arrive for a press briefing about the ongoing situation with the COVID-19 coronavirus outbreak, inside 10 Downing Street in London.  (AP Photo/Matt Dunham)

March 17, 2020: Britain's Chancellor Rishi Sunak, left, and Prime Minister Boris Johnson arrive for a press briefing about the ongoing situation with the COVID-19 coronavirus outbreak, inside 10 Downing Street in London.  (AP Photo/Matt Dunham)

Johnson's move had raised questions and concerns about how the British government would run, especially considering that the line of succession is not explicitly set out in the unwritten British Constitution.

Foreign Secretary Dominic Raab had been deputized "where necessary" in Johnson’s absence and was conducting daily cabinet meetings as well as daily government briefings to the public in Johnson's place.

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Britain has been in an effective lockdown since March 23 and the government is set to extend the restrictions sometime next week.

As of Monday, the United Kingdom recorded 85,212 confirmed cases, with at least 10,612 deaths, according to Johns Hopkins University.

Fox News' Travis Fedschun, Adam Shaw and the Associated Press contributed to this report.