British Prime Minister Boris Johnson remained in stable condition on Tuesday evening in London, but will stay in intensive care for "close monitoring" after suffering a deterioration of his COVID-19 symptoms yesterday, his spokesperson said. He added that Johnson remains in good spirits.

Earlier on Tuesday, the spokesperson told reporters the prime minister, who was admitted to hospital on Sunday, was receiving standard oxygen treatment and was breathing without any other assistance. He was still conscious and did not require a mechanical ventilator, his office said.

"The prime minister has been stable overnight and remains in good spirits. He is receiving standard oxygen treatment and breathing without any other assistance. He has not required mechanical ventilation, or non-invasive respiratory support," the spokesperson, who traditionally speaks without his name being published, told reporters.

Johnson's personal battle with the virus has shaken the British government just as the United Kingdom enters what scientists say is likely to be one of the most deadly weeks of the pandemic, which has killed 70,000 people worldwide.

Johnson, 55, was admitted to St Thomas' Hospital across the River Thames from the House of Commons late on Sunday after suffering persistent coronavirus symptoms, including a high temperature and a cough, for more than 10 days.

A man with a mask reads a newspaper with the headline 'PM in intensive care,' outside St Thomas' Hospital in central London on Tuesday. (Kirsty Wigglesworth/The Associated Press)

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 and was moved to intensive care in case he needed to be put on a ventilator.

"The prime minister is in intensive care, being looked after by his medical team, receiving the very, very best care from the team at St Thomas' and our hopes and prayers are with him and with his family," Cabinet Office Minister Michael Gove said earlier Tuesday.

"We all hope that he can be restored to health as quickly as possible, and that's all we're thinking about at the moment," said Gove, who also announced he was self-isolating after a family member developed mild coronavirus symptoms.

Former British prime minister David Cameron predicted Johnson would "come through this," calling him resilient, while Canadian Prime Minister Justin Trudeau was among a number of international world leaders wishing him a speedy recovery.

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

Government clarifies line

Foreign Secretary Dominic Raab is currently standing in for Johnson. British Finance Minister Rishi Sunak would take responsibility for leading the country if Raab were unable to, the Downing Street spokesperson said.

"There is an established order of precedence," Johnson's spokesperson said. "In line with the order of precedence, the chancellor [Sunak] would follow from the Foreign Secretary."

Finance Minister Rishi Sunak, right, is seen October 2019 with Britain's Foreign Secretary Dominic Raab, centre, and Britain's Secretary of State for Scotland Alister Jack. Raab has been deputized in the wake of Johnson's illness, with a government spokesperson indicating Sunak would be next in line after Raab to temporarily head the government if necessary. (Henry Nicholls/Reuters)

Johnson's move to intensive care added to the sense of upheaval that the coronavirus crisis has wrought after its spread caused widespread panic, sowed chaos through financial markets and prompted the virtual shutdown of the global economy.

"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," a sombre Raab said Monday.

Raab, 46, takes the helm at a pivotal time: The official death toll in the U.K. currently stands at 5,655, and last week the health minister said the deadliest peak for deaths was projected to be Easter Sunday, April 12.

Earlier today The Queen sent a message to Carrie Symonds and to the Johnson family. Her Majesty said they were in her thoughts and that she wished the Prime Minister a full and speedy recovery. <a href="https://t.co/Mo1SgAd9wh">pic.twitter.com/Mo1SgAd9wh</a> —@RoyalFamily

The U.K. is in a state of virtual lockdown, a situation due to be reviewed early next week, and some ministers have suggested it might need to be extended because some people were flouting the strict rules.

There have also been calls for ministers to detail what the exit plans were from the shutdown, which has hammered the world's fifth-biggest economy after the government ordered restaurants, bars, and nearly all shops to close and told people to stay at home to curb the spread of the virus.

Johnson's fiancée had symptoms

Johnson tested positive for the virus on March 26, the first leader of a major power to announce that he had been infected.

After 10 days of isolation in an apartment at Downing Street, he was admitted to hospital. He was last seen in a video message posted on Twitter on Friday when he looked weary.

Johnson is seen with his partner Carrie Symonds on March 7 when both attended a rugby match at Twickenham Stadium in London. (Toby Melville/Reuters)

Johnson, who is not a smoker, said recently that he wanted to lose weight. He plays tennis and while mayor of London used to cycle around the capital. Johnson's pregnant 32-year-old fiancée, Carrie Symonds, also had symptoms but said on Saturday she was feeling better.

Downing Street said repeatedly on Monday that Johnson remained in charge and was reading documents, but the move to intensive care revealed the gravity of his condition.

"There is no doubt this turn of events means Boris Johnson is extremely sick," said Derek Hill, a professor of medical imaging at University College London.

The face of the 2016 Brexit campaign, Johnson won a resounding election victory in December before leading the U.K. out of the European Union on Jan. 31.