UK Prime Minister Boris Johnson was moved into an intensive-care unit after his condition deteriorated on Monday.

Johnson, who was admitted to hospital on Sunday with "persistent" symptoms of COVID-19, the illness caused by the coronavirus, was moved into intensive care at about 7 p.m. GMT on Monday, his spokesman 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," the spokesman 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."

Dominic Raab, the UK's foreign secretary, will stand in for Johnson while he is treated.

"The PM has asked Foreign Secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary," Johnson's spokesman said.

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

In a statement to journalists, Raab said that "there is an incredibly strong team spirit behind the Prime Minister and we're making sure that we get all the plans he has instructed us to deliver implemented as soon as possible."

Cabinet Secretary Sir Mark Sedwill broke the news to the rest of the Cabinet via a video call, Sky News reported.

Johnson spent over a week in self-isolation after developing symptoms of COVID-19 before he was taken to the hospital on Sunday for tests.

As of Monday morning, Downing Street had insisted that the prime minister was still running the UK government. Johnson's spokesman had said the prime minister was in "good spirits" and was "comfortable" after spending the night at the central London hospital.

However, his condition significantly worsened later in the day, forcing doctors to move him into intensive care.

Johnson received warm wishes from around the world and across the political spectrum.

President Trump told a press briefing that "Americans are all praying for his recovery," adding that "he's been a very good friend."

Keir Starmer, the leader of the opposition Labour Party, described the news as "terribly sad."

"All the country's thoughts are with the Prime Minister and his family during this incredibly difficult time," he tweeted.

Former Prime Minister David Cameron tweeted: "Thinking of @BorisJohnson and his family tonight. Get well soon. You are in great hands and we all want you safe, well and back in @10DowningStreet."

Johnson's aides were 'increasingly worried' about his health

Johnson. Getty

Johnson's spokesman earlier on Monday refused to confirm or deny that the prime minister had pneumonia.

The Times of London reported on Monday morning that the prime minister had been given oxygen on Sunday night.

His spokesman did not deny that report on Monday but did insist that separate claims in Russian state media that he was placed on a ventilator were "disinformation."

Downing Street had repeatedly said Johnson was experiencing "mild symptoms" of COVID-19.

However, aides have become "increasingly worried" about the prime minister's health, The Sun reported, with government aides telling the publication Johnson was heard "coughing and spluttering" on conference calls.

The Guardian said it had been told that "Johnson was more seriously ill than either he or his officials were prepared to admit." It described sources as saying Johnson "was being seen by doctors who were concerned about his breathing."

A tweet from Johnson's account earlier Monday afternoon said he was "in good spirits."