British Prime Minister Boris Johnson is being treated for worsening coronavirus symptoms in an intensive care unit, with his foreign minister deputising for him as the nation tackles the COVID-19 crisis.

The UK has so far recorded 52,279 positive cases of the coronavirus, 5385 people have lost their lives and 287 patients have recovered.

Mr Johnson, 55, was admitted to St Thomas’ Hospital in central London late on Sunday after suffering persistent coronavirus symptoms, including a high temperature, for more than 10 days.

His condition rapidly deteriorated over the next 24 hours, and he was moved to an intensive care unit, where the most serious cases are treated. Although he had received oxygen, his office said he was still conscious.

Downing Street said the move to intensive care was “a precaution should he require ventilation to aid his recovery”.

Britain has no formal succession plan should a prime minister become incapacitated.

“The PM has asked Foreign Secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary,” Downing Street said.

Just hours earlier, Johnson’s office said he was in good spirits and Mr Raab had told a news conference that the prime minister was still running the government.

“The government’s business will continue,” a sombre Mr Raab, 46, told reporters, saying Johnson was in the safe hands of a brilliant medical team.

“The focus of the government will continue to be on making sure that the prime minister’s direction, all the plans for making sure that we can defeat coronavirus and can pull the country through this challenge, will be taken forward.”

Tweet from @BorisJohnson

Mr Johnson, who is expecting a child with partner Carrie Symonds, has had persistent coronavirus symptoms since testing positive for the infection on March 26. 

He was working from an apartment at Downing Street, and until Tuesday morning (Australian time) his office had insisted he was continuing work as usual.

Just hours later, the government released a less optimistic statement and confirmed that Mr Raab had been deputised to stand in for the PM. 

https://twitter.com/RobertBuckland/status/1247246823099686912

Across Britain and the globe, politicians from all sides have taken to social media to wish Mr Johnson well.

Buckingham Palace said Queen Elizabeth had been kept informed by Downing Street and US President Donald Trump said all Americans were praying for his recovery.

“He’s been really something very special – strong, resolute, doesn’t quit, doesn’t give up,” Mr Trump told a news briefing, adding he had asked two pharmaceutical firms developing potential COVID-19 therapies to get in touch with the British government to offer their services.

“We’ve contacted all of Boris’ doctors and we’ll see what is going to take place,” he said.

“But they are ready to go.”

Mr Johnson and his health minister tested positive last month and chief medical adviser Chris Whitty self-isolated.

Mr Johnson’s pregnant 32-year-old fiancee, Carrie Symonds, also had symptoms but said on Saturday she was feeling better.

German Chancellor Angela Merkel wrote alongside a love heart and prayer emoji: “Get well soon Boris.. I wish you a speedy recovery..”

“Thoughts and prayers with Boris, @carriesymonds and all their family. This is such a terrible virus that we all must take seriously,” wrote MP Ian Blackford.

“This must be so scary. I hope the government can now stop this pretence that all is fine and concentrate on getting him well and reassure the country,” MP Layla Moran wrote.

-with AAP