Johnson was admitted to hospital on Sunday night for tests after suffering persistent COVID-19 symptoms, including a high temperature, for more than a week.

The 55-year-old PM tested positive for the virus last month and was isolating in an apartment at Downing Street.

His condition worsened on Monday and he was subsequently moved into the intensive care unit at the St Thomas Hospital in central London.

Unprecedented

Johnson is the most high-profile world leader with the disease that has spread rapidly across the globe, and his transfer to intensive care is unprecedented during a national emergency.

US President Donald Trump led world leaders in wishing Johnson a successful recovery. Other messages flooded in from the European Union and NATO to the World Health Organisation.

The British government was criticised for initially refusing to follow other European countries in requiring people to stay home as the virus spread.

Johnson himself said in early March that he was still shaking hands with people.

But two weeks ago he ordered a nationwide lockdown and Britain is now in the grip of a serious outbreak. 

Over 50,000 cases and more than 5,000 deaths have been recorded so far, with a latest daily toll of 439.

Health Secretary Matt Hancock and the heir to the throne, Prince Charles, have both been infected with coronavirus, although they have since recovered.

The queen, 93, made a rare public address on Sunday night, evoking the spirit of World War II and urging Britons to stay united, just before Johnson went into hospital.

"We will be with our friends again, we will be with our families again, we will meet again," she said in a rare televised address watched by 23 million people.

Buckingham Palace said she was being kept informed of developments.

Working non-stop

Housing minister Robert Jenrick told BBC television that Johnson had been working "phenomenally hard" during the crisis, adding that he would be finding it "very frustrating" to be ill.

Johnson is not known to have any underlying health issues, although he has struggled with his weight, but some questioned if he should have taken more time off.

Junior health minister Nadine Dorries, who also had coronavirus but has recovered, added: "Many with #COVID19 are felled by fatigue/temperature and use isolation to sleep and recover. 

"Boris has risked his health and worked every day on our behalf to lead the battle against this vile virus."

James Gill, clinical lecturer at Warwick University medical school, said Johnson's condition "must underscore for everyone, across the world, how indiscriminate this virus is, ignoring class, character, wealth and position".

Persistent symptoms

Trump said he was "hopeful and sure" Johnson would recover, calling the prime minister "a friend of mine" and a "great leader".

Johnson's pregnant partner, Carrie Symonds, moved out of Downing Street after some staff fell ill. 

But she said on Saturday she had just spent a week in bed with symptoms, although she has not been tested.

Johnson's spokesman would not confirm a report in The Times newspaper that the prime minister had been given oxygen treatment.

"Doctors will be monitoring important vital signs such as oxygen saturations," said Rupert Beale, group leader at the cell biology of infection laboratory at the Francis Crick Institute.

He said they would also check Johnson's blood to "see what the immune response to the virus looks like, and to assess liver and kidney function." They may also perform an electrocardiogram to check the heart.

eNCA/AFP