London, United Kingdom - UK Prime Minister Boris Johnson, who is battling worsening coronavirus symptoms, was stable during his first night in an intensive care unit at a London hospital, the PM's spokesman said on Tuesday.

"The prime minister has been stable overnight and remains in good spirits. He is receiving standard oxygen treatment and breathing without any other assistance. He has not required mechanical ventilation or non-invasive respiratory support," the spokesman said, adding that Johnson did not have pneumonia.

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Johnson, 55, first announced he had tested positive for the virus on March 27 and went into self-isolation.

After failing to shake off a high fever for more than a week, he was admitted to Guy's and St Thomas' hospital in central London on Sunday from where he continued to lead the government until he was moved into intensive care on Monday evening when his condition deteriorated.

"Over the course of [Monday] 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," Number 10 said in a statement.

Prime Minister Boris Johnson on a screen as he remotely chaired the morning novel coronavirus COVID-19 meeting by video link, in Downing Street in central London. [File: Andrew Parsons/10 Downing Street/ AFP]

Johnson has asked Foreign Secretary Dominic Raab to deputise where necessary.

In Tuesday's daily afternoon briefing, Raab told reporters that the prime minister was breathing without assistance and had not required respiratory support.

"He remains in good spirits," said Raab. "And in keeping with usual clinical practice, his progress continues to be monitored closely.

"I'm confident he will pull through. If there is one thing I know about this prime minister, he's a fighter."

It was also announced on Tuesday afternoon that, as of 16:00 GMT on Monday, 6,159 people in the UK had died from coronavirus, according to the health ministry. More than 55,000 people have so far tested positive for the virus.

The PM's spokesman on Tuesday said Raab and the cabinet had the authority to decide on national security issues in Johnson's absence, but that Raab did not currently have the power to hire or fire ministers.

If Raab became unable to lead, Rishi Sunak, the chancellor, would lead government, said the spokesman.

Before being moved within the hospital, Johnson tweeted: "Last night, on the advice of my doctor, I went into hospital for some routine tests as I'm still experiencing coronavirus symptoms. I'm in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe.

"I'd like to say thank you to all the brilliant NHS staff taking care of me and others in this difficult time. You are the best of Britain. Stay safe everyone and please remember to stay at home to protect the NHS and save lives."

Queen Elizabeth II, who recently gave a rare address to the nation amid the coronavirus crisis, is being kept informed about Johnson's health.

While doctors attempted to help Johnson recover on Tuesday, world leaders paid tribute as political figures in the United Kingdom across the spectrum set differences aside to send messages of support. US President Donald Trump said: "I've gotten to know him [Johnson]. He's just such an incredible guy.

"It was just so shocking to see that because you know what that means - intensive care is a big deal with regard to what we're talking about. That's a very big deal. Very scary deal.

"All my support for Boris Johnson, his family and the British people at this difficult time."

Canadian Prime Minister Justin Trudeau said he wished "for a full and speedy recovery" for his British counterpart.

"My thoughts are with you and your family right now. Hope to see you back at Number 10 soon."

Keir Starmer, the new leader of the opposition Labour Party, tweeted: "Terribly sad news. All the country's thoughts are with the Prime Minister and his family during this incredibly difficult time."

Starmer's predecessor Jeremy Corbyn said: "My thoughts are with Boris Johnson and his family.

"Thanks to the NHS staff for their hard work and dedication."