British Prime Minister Boris Johnson was moved to an intensive care unit on Monday after his coronavirus symptoms worsened, though his Downing Street office said he was still conscious.

"Since Sunday evening, the prime minister has been under the care of doctors at St. Thomas' Hospital, in London, after being admitted with persistent symptoms of coronavirus," a statement released by his office said.

"Over the course of this 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.

"The PM has asked Foreign Secretary Dominic Raab, who is the first secretary of state, to deputize for him where necessary," Johnson's office said. "The PM remains conscious at this time."

WATCH l Scene around St. Thomas' Hospital, where Boris Johnson is in intensive care:

Prime Minister Boris Johnson, who has coronavirus, is said to have worsening symptoms 0:50

Downing Street said Johnson had been moved to the intensive care unit — where the most serious cases are treated — as "a precaution should he require ventilation to aid his recovery." Johnson had been undergoing tests after suffering persistent symptoms, including a high temperature, for more than 10 days.

Downing Street had said he was in good spirits and still in charge, though his condition deteriorated in the early evening and he was transferred at about 6 p.m. GMT.

No formal succession plan

With only an unwieldy collection of sometimes ancient and contradictory precedents to go by, there is no formal succession plan should the British prime minister become incapacitated.

Raab, 46, chaired the government's emergency COVID-19 meeting on Monday. At the daily coronavirus media briefing, he said Johnson was being "regularly updated," but admitted he had not spoken to him since Saturday.

After Johnson's admission to intensive care, Raab said the British government will continue to carry out the prime minister's efforts to defeat the COVID-19 outbreak.

"The prime minister is in safe hands … and 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," Raab said.

Sending my best wishes to Prime Minister <a href="https://twitter.com/BorisJohnson?ref_src=twsrc%5Etfw">@BorisJohnson</a> 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. —@JustinTrudeau

World leaders and other political figures, including Canadian Prime Minister Justin Trudeau and U.S. President Donald Trump, were quick to express their get-well wishes for Johnson.

"Sending my best wishes to Prime Minister Boris 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," Trudeau tweeted, referring to the street number of the U.K. prime minister's official residence and office.

WATCH l What Boris Johnson's move into intensive care means:

U.K. Prime Minister Boris Johnson's move into intensive care means his 'deterioration has been rapid,' says CBC medical contributor Dr. Peter Lin. 11:32

Trump spoke of Johnson's condition soon afterward during a White House news conference.

"He's been really something very special — strong, resolute, doesn't quit, doesn't give up," he said, before 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."

Tedros Adhanom Ghebreyesus, director-general of the World Health Organization (WHO), also sent words of encouragement via Twitter.

"I am thinking of my friend Boris Johnson tonight," Tedros wrote. "And sending my and WHO's heartfelt good wishes as he battles the coronavirus."

'Huge concern'

Linda Bauld, a professor of public health at the University of Edinburgh, said the prime minister's admission to intensive care was "of huge concern."

"[It] illustrates just how indiscriminate this virus is. Anyone anywhere, including the most privileged in our society, can be affected and can become seriously ill," Bauld said.

"It is imperative now, more than ever, that the rest of us comply with government guidelines to stay at home and not put others at risk."

Earlier on Monday, a spokesperson said Johnson, 55, had a "comfortable" first night in hospital and was in "good spirits" while continuing to work. The prime minister had also thanked "the brilliant [National Health Service] staff taking care of me and others in this difficult time."

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. <br><br>Stay safe everyone, and please remember to stay at home to protect the NHS and save lives. —@BorisJohnson

Johnson's pregnant 32-year-old fiancée, Carrie Symonds, also had symptoms but said Saturday she was feeling better.

Prior to his transfer to the ICU, experts said a person of Johnson's age with COVID-19 symptoms after 10 days was likely to be assessed for their oxygen levels, lung, liver and kidney functions, and undergo an electrocardiogram heart check.

"Clearly the prime minister is finding it difficult to shake this thing off," said Jonathan Ball, a professor of molecular virology at Nottingham University.

"What it does show is how difficult it is to predict how this infection will develop, and whilst most people will experience nothing more than an annoying cold, for others this can develop into a serious and sometimes life-threatening disease."

Medics said that patients with COVID-19 can deteriorate after about 10 days, with some developing pneumonia. National Health Service guidelines advise those who are self-isolating not go to hospital unless they develop new symptoms or become sufficiently unwell.

In terms of Britain's fight in dealing with the COVID-19 outbreak, the prime minister's office said Britain now has 10,000 ventilators in its health care system, confirming the number had increased after deliveries from suppliers.

As of 4 p.m. Sunday, the country had 51,608 confirmed coronavirus cases and a death toll of 5,373.