U.K. Prime Minister Boris Johnson is being treated in the intensive care unit of St. Thomas' Hospital in London due to increasingly severe coronavirus symptoms.

What they're saying: Cabinet minister Michael Gove told LBC radio on Tuesday morning Johnson was not on a ventilator. "The prime minister has received some oxygen support and he is kept under, of course, close supervision," he said.

The backdrop: Johnson was admitted to the hospital on Sunday for what Downing Street called "routine tests" because his condition had not improved 10 days after he tested positive for the virus. His condition had since "worsened" and he was moved to the ICU on Monday, according to a statement.

Foreign Secretary Dominic Raab will step into his place "where necessary," it said.

Johnson remains conscious at this time and was moved to the ICU as a precaution in case a ventilator is needed, according to Downing Street.

The big picture: Johnson, 55, was the first world leader to announce he had tested positive for the virus, on March 27.

He continued to lead the government's response to the coronavirus and held meetings and briefings via video conference.

After he entered the hospital on Sunday, Downing Street faced questions as to why Johnson had not handed authority to Raab — the second-highest-ranking government official — on an interim basis.

Johnson's fiancee, Carrie Symonds, said on Saturday that she had been self-isolating with coronavirus symptoms but was beginning to feel better. Symonds is pregnant with the couple's first child.

Full statement:

“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.

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 PM has asked Foreign Secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary.

The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication.”

— Downing Street spokesperson

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Editor's note: This article has been updated with more details, including Gove's comments.