London (CNN) UK Prime Minister Boris Johnson was moved to an intensive care unit Monday after his condition with coronavirus symptoms "worsened," his office has said.

Johnson, 55, was first admitted to St Thomas' Hospital in London on Sunday evening for what he said were "routine tests," saying on social media that he was in "good spirits."

But the apparent change in the leader's health has shocked the nation, with politicians from his party and the opposition voicing their support for the leader, as the country battles an outbreak that has killed more than 5,000 people.

"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," a Downing Street spokesperson said.

According to Downing Street officials, Johnson remains conscious and has been moved to the intensive care unit as a precaution, in case he requires ventilation.

The move strikes a sharp contrast to earlier reports on Johnson's condition: During his period of self-isolation, Downing Street had said that Johnson's symptoms were "mild," and on Sunday said his hospitalization was just a " precautionary step ."

The Prime Minister's spokesperson had said earlier on Monday that Johnson spent a "comfortable" night in hospital, but would not comment on reports Johnson was given oxygen. Officials are no longer describing his symptoms as mild.

Foreign Secretary Dominic Raab, who is the First Secretary of State and the most senior Cabinet minister, said Johnson had asked him to deputize for the leader where necessary.

Raab chaired Monday's regular morning coronavirus briefing and made a televised statement saying that the government will continue Johnson's direction in responding to the outbreak.

"The government's business will continue and the Prime Minister is in safe hands with a brilliant team at St Thomas's hospital. 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," he said.

He added there was an "incredibly strong team spirit" in government behind the Prime Minister.

Johnson was moved to the intensive care unit at around 7 p.m., Downing Street said. The decision was made by the leader's medical team after his condition worsened over the afternoon on Monday.

The United Kingdom's death toll of above 5,000 is one of the highest in the world. The government has been scrambling to implement widespread testing as evidence grows that asymptomatic people could carry and spread the virus.

'Together we will beat this,' Johnson said

Johnson was hospitalized Sunday because he was still suffering coronavirus symptoms 10 days after testing positive for the virus.

The British leader announced on March 27 that he had been infected and was, at the time, suffering mild symptoms including a cough and fever.

At the time, he vowed to continue leading the nation's response to the outbreak and hold meetings via video-conference, saying in a video: "Together we will beat this."

British politicians from Johnson's ruling Conservative Party, as well as the main opposition Labour Party, have united in their wishes for the Prime Minister's recovery.

Treasurer Rishi Sunak said on Twitter that his thoughts were with Johnson and his pregnant fiancée, Carrie Symonds.

"I know he'll be getting the best care possible and will come out of this even stronger," he said.

Symonds has also experienced Covid-19 symptoms but said over the weekend she was "on the mend."

The leader of the country's House of Commons, Lindsay Hoyle, said Johnson's admission to intensive care was "terrible news."

"I know the thoughts and prayers of everyone across the House are with the Prime Minister and his family right now. We all wish him a speedy recovery."

A still from a video that UK Prime MInister Boris Johnson uploaded on March 27 to announce he had been infected.

Keir Starmer, the newly elected leader of the main opposition Labour Party said the development was "Terribly sad news."

"All the country's thoughts are with the Prime Minister and his family during this incredibly difficult time."

And former Prime Minister Theresa May, who was replaced by Johnson in the country's Brexit crisis, also sent her wishes to the leader on Twitter.

"My thoughts and prayers are with @BorisJohnson and his family as he continues to receive treatment in hospital. This horrific virus does not discriminate. Anyone can get it. Anyone can spread it. Please #StayHomeSaveLives," May wrote.

French President Emmanuel Macron also sent his wishes. "I send all my support to Boris Johnson, to his family and to the British people at this difficult moment. I wish him a speedy recovery at this testing time."

US President Donald Trump responded to the news during a daily press conference, and said that his administration had been in contact with Johnson's doctors.

"I also want to send best wishes to a very good friend of mine and a friend of our nation Prime Minster Boris Johnson. We're very saddened to hear that he was taken into intensive care this a little while ago," Trump said. "He's been a really good friend. He's been really something very special. Strong. Resolute."

"We've contacted all of Boris's doctors, and we'll see what is going to take place, but they are ready to go," Trump continued. "When you are brought into intensive care that gets very, very serious."

"We are working with London with respect to Boris Johnson," he concluded.

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. — Boris Johnson #StayHomeSaveLives (@BorisJohnson) April 6, 2020

Before being moved to intensive care, Johnson had said he was keeping in touch with his team and thanked staff at Britain's National Health Service for taking care of him.