British Prime Minster Boris Johnson leaves 10 Downing Street for Prime Minister's Questions, in London, Britain, Feb. 26, 2020.(Photo by Tim Ireland/Xinhua)

British Prime Minister Boris Johnson was fighting worsening coronavirus symptoms in an intensive care unit on Tuesday, leaving his foreign minister to lead the government's response to the accelerating outbreak.

In a statement on Tuesday, a Downing Street spokesman said: "The prime minister has been stable overnight and remains in good spirits. He is receiving standard oxygen treatment and is breathing without any other assistance," BBC reported on Tuesday.

Johnson's personal battle with the virus has shaken the British government just as the UK enters what scientists say is likely to be one of the deadliest weeks of the pandemic.

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.

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.

Johnson is not on a ventilator but has had some oxygen support and if his condition changes the government will make an official statement, Cabinet Office Minister Michael Gove said on Tuesday.

"He's not on a ventilator no," Gove told LBC radio. "The prime minister has received some oxygen support and he is kept under, of course, close supervision."

"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," Gove noted.

While Britain has no formal succession plan should a prime minister become incapacitated, Johnson had asked Foreign Secretary Dominic Raab to deputize for him "where necessary," Downing Street said.

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

Reuters / BBC