UK Prime Minister Boris Johnson will remain in hospital "as long as necessary" on Monday after being admitted on Sunday for "persistent symptoms of coronavirus." Johnson tested positive for the coronavirus 10 days ago and continues to experience a high temperature and a cough.

He was reportedly treated with oxygen after arriving at St Thomas' Hospital in London on Sunday night. Aides reportedly had become "increasingly worried" about his health because of his appearance and symptoms.

Johnson was "more seriously ill than either he or his officials were prepared to admit," according to the Guardian, which reported a source suggesting Johnson "was being seen by doctors who were concerned about his breathing." Downing Street would not comment on whether he was suffering from pneumonia.

What will happen if Boris Johnson becomes too ill to remain prime minister.

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UK Prime Minister Boris Johnson will remain in hospital on Monday after being admitted for "persistent symptoms of coronavirus," 10 days after first testing positive for it.

The prime minister was admitted to St Thomas' Hospital in Westminster at 8 p.m. on Sunday on the advice of his doctor after continuing to exhibit a high temperature.

A spokesperson insisted on Monday that Johnson's hospital admission was not an "emergency" measure but had merely been for precautionary reasons to carry out tests.

They refused to confirm or deny that the prime minister was suffering from pneumonia.

But the Times of London newspaper reported that the prime minister was treated with oxygen on arrival.

His spokesman would not deny that report on Monday but insisted that separate claims in Russian state media that he is on a ventilator were "disinformation."

Downing Street has previously repeatedly insisted that Johnson was experiencing only "mild symptoms" of the virus.

However, aides have reportedly become "increasingly worried" about the prime minister's health, according to multiple reports, with Johnson heard "coughing and spluttering" his way through conference calls.

Johnson was "more seriously ill than either he or his officials were prepared to admit," the Guardian said, reporting a source suggesting that Johnson "was being seen by doctors who were concerned about his breathing."

The Sun newspaper reported a Downing Street source suggesting that Johnson would remain in hospital "as long as necessary."

Johnson's personal Twitter account carried a message purportedly from the prime minister on Monday, suggesting he was in "good spirits."

Asked about the prime minister's condition on Monday, the Housing Secretary Robert Jenrick told the BBC that Johnson was "still very much in charge of the government."

"We hope that as a result of these tests he will be able to come back to Downing Street as soon as possible," Jenrick said. "He's been working extremely hard leading the government and being constantly updated. That's going to continue."

He added: "I'm sure this is very frustrating for him, for somebody like Boris who wants to be hands [on] running the government from the front, but nonetheless he's still very much in charge of the government."

Yet the Foreign Office Minister James Duddridge called on Johnson to "rest, look after yourself and let the others do the heavy lift."

—James Duddridge MP #StayHomeSaveLives (@JamesDuddridge) April 5, 2020

A spokesperson for the prime minister said on Sunday, "On the advice of his doctor, the Prime Minister has tonight been admitted to hospital for tests.

"This is a precautionary step, as the Prime Minister continues to have persistent symptoms of coronavirus 10 days after testing positive for the virus.

"The Prime Minister thanks NHS staff for all of their incredible hard work and urges the public to continue to follow the Government's advice to stay at home, protect the NHS and save lives."

Johnson's girlfriend, Carrie Symonds, who is pregnant with their child, is also suffering from the virus.

She tweeted on Saturday: "I've spent the past week in bed with the main symptoms of coronavirus. I haven't needed to be tested and, after seven days of rest, I feel stronger and I'm on the mend."

Trump says Johnson's hospital admission is 'a big move'

President Trump said he had spoken to the British ambassador in Washington.

"Hey, it's a big move going to the hospital — it's a big thing," Trump told journalists. "He's a great gentleman so I — I just hope he's OK."

He added: "I want to express our nation's well wishes to Prime Minister Boris Johnson as he wages his own personal fight with the virus. All Americans are praying for him. He's a friend of mine. He's a great gentleman and a great leader."

Dominic Raab would act as deputy for Johnson

Foreign Secretary Dominic Raab. Getty

The Foreign Secretary Dominic Raab, who has been nominated as Johnson's "designated survivor," was scheduled to chair an emergency coronavirus meeting in Johnson's absence on Monday morning.

Downing Street insisted the prime minister remained in charge of the government.

If Johnson's condition were to deteriorate, then Raab, who has twice tested negative for the coronavirus, could become the de facto prime minister while Johnson receives further treatment.

The decision to give Raab the job reportedly upset some other members of Johnson's Cabinet.

One unnamed minister told The Sunday Times last week: "If Boris can't do his job because he is incapacitated, a lot of people think that Michael [Gove] should be running the show, not Raab. One of these people is Michael, of course."