Johnson had earlier asked Foreign Secretary Dominic Raab to temporarily take over the prime minister’s duties where necessary.

A statement from 10 Downing Street on Monday said: “Over the course of this afternoon (Monday), 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 St Thomas’ Hospital, just across the Thames from Westminster.

This came a day after Johnson was sent to hospital from his home, where he had been in self-isolation since being diagnosed with Covid-19 on March 26. He wrote on Twitter that he had checked in “for some routine tests as I’m still experiencing coronavirus symptoms”.

The prime minister remained conscious at that, his spokesman said, adding that he had been moved to the ICU as a precaution should he require ventilation to aid his recovery.

Raab, who has taken charge of the British government, said Johnson is now in the critical care unit, adding: “The prime minister is in safe hands with the brilliant team at St Thomas' Hospital.”

UK coronavirus strategy: ‘keep calm and wash your hands’

Concerning the cabinet, Raab said there was an “incredibly strong team spirit” behind Johnson, in a bid to reassure the public that “the government's business will continue”.

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

Buckingham Palace said that Queen Elizabeth, who remains at Windsor Castle, had been “kept informed” on the prime minister’s condition and his transfer to intensive care.

Liu Xiaoming, the Chinese ambassador to Britain, joined a long line of diplomats and world leaders in wishing Johnson a speedy recovery.

On his personal Twitter account, Liu said: “Our thoughts are with Prime Minister Boris Johnson. Let’s hope and pray that he will get well soon.”

UK coronavirus ‘lockdown’ confusion as nation braces for wave of infections

US President Donald Trump said he asked “leading companies” to contact officials in London about therapies that could help Johnson.

He called them complex treatments recently developed by unnamed firms that have dealt with “ebola, Aids, others”.

“We’ll see if we can be of help. We’ve contacted all of Boris’ doctors,” Trump told reporters.

The US president also offered his “best wishes to a very good friend of mine, and a friend to our nation,” adding that “Americans are all praying for his recovery”.

French President Emmanuel Macron said he wished him to “overcome this ordeal quickly”.

Canadian Prime Minister Justin Trudeau, whose wife recently recovered from Covid-19, sent his best wishes to Johnson for a full and speedy recovery. “My thoughts are with you and your family right now. Hope to see you back at Number 10 soon,” he said.

Johnson's fiancée, Carrie Symonds, is expecting the couple’s baby in early summer. She said on Saturday she had spent a week in bed with the main coronavirus symptoms, but added that she had not been tested.

Britain had reported 5,373 deaths from more than 52,000 confirmed Covid-19 cases by Monday, making it one of the worst-affected countries. But government experts estimate that hundreds of thousands of people are infected.

Many health experts have criticised the government’s slow response to the crisis, the low level of testing for the virus and the poor provision of intensive care beds, ventilators and protective equipment.

Adding to the pressure on Johnson, some right-wingers in his ruling Conservatives have warned that Britain faces economic ruin unless it eases its near-lockdown soon.

Additional reporting by Politico and DPA