U.K. Prime Minister Boris Johnson was moved to intensive care as his coronavirus symptoms worsened over the course of several hours Monday, according to a statement from the government.
Foreign Secretary Dominic Raab will temporarily take over the prime minister's duties while Johnson is hospitalized, the government said.
"Since Sunday evening, the Prime Minister has been under the care of doctors at St Thomas' Hospital, in London, after being admitted with persistent symptoms of coronavirus," a spokesman for No. 10 Downing St. said in a statement.
"Over the course of this afternoon, the condition of the Prime Minister has worsened and, on the advice of his medical team, he has been moved to the Intensive Care Unit at the hospital," the spokesman said.
"The PM has asked Foreign Secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary," the spokesman added. "The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication."
Johnson was conscious when he was transferred to the ICU around 7 p.m. GMT (3 p.m. ET). Johnson's medical team made the decision to move him to that unit as a precaution in case he needed ventilation.
Johnson, 55, announced on March 27 that he had contracted the disease and was experiencing "mild symptoms," including "a temperature and a persistent cough."
Ten days later, Johnson entered a hospital "for some routine tests as I'm still experiencing coronavirus symptoms," according to a tweet Monday morning from his official account.
"I'm in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe," Johnson said in the tweet.
Johnson's diagnosis made him the first major governmental leader known to have contracted the disease.
A total of 5,373 Britons have died after testing positive for the coronavirus, the U.K.'s Department of Health and Social Care said Monday, up 439 from the previous day.
It said that of the 208,837 people tested, 51,608 have tested positive for the virus.
Two days before Johnson revealed that he had tested positive for the coronavirus, the British royal household announced that Prince Charles, the 71-year-old heir to the throne, had contracted the disease.
Raab told reporters later Monday that Johnson's team will make sure to implement his plans for responding to the coronavirus as soon as possible.
The U.K. had been accused by experts of responding too slowly to the pandemic. Johnson imposed a suite of nationwide lockdown measures on March 20, after other European nations had already taken extreme measures to contain the rapid spread of the disease.
President Donald Trump, who said he has twice tested negative for COVID-19, has offered words of support for the prime minister during White House press briefings on the virus.
Trump said of Johnson on Monday afternoon that he was "very saddened to hear that he was taken into intensive care a little while ago."
"He's been really something very special," Trump said. "Strong, resolute, he doesn't quit, doesn't give up."
"But when you get brought into intensive care, that gets very very serious," Trump added.
Ivanka Trump, the president's daughter and advisor, tweeted that her "thoughts and prayers" are with Johnson and his family.
"Godspeed Mr. Prime Minister!" she tweeted.
-- CNBC's Katrina Bishop contributed to this report.