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

Britain has no formal succession plan should the Prime Minister become incapacitated, but Johnson, 55, asked foreign secretary Dominic Raab to deputise for him. Johnson was admitted to hospital on Sunday night and had been undergoing tests after suffering persistent coronavirus symptoms, including a high temperature, for more than 10 days.

Downing Street had said he was in good spirits and still in charge, though at about 1800 GMT, he was moved to an intensive care unit - where the most serious cases are treated - at St Thomas' hospital, across the River Thames from the Houses of Parliament in central London. Johnson had received oxygen, a source 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," a spokesman for his office said. The PM has asked foreign secretary Dominic Raab, who is the first secretary of state, to deputise for him where necessary," Downing Street said.

UK foreign secretary Dominic Raab has taken charge as Johnson was moved to the ICU at St Thomas' Hospital in London on Monday.

"The PM remains conscious at this time." Downing Street said he had been moved to the intensive care unit as "a precaution should he require ventilation to aid his recovery". Johnson, 55, tested positive for the virus on March 26. His spokesman had earlier refused to answer directly after being asked whether he had pneumonia.

"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,” Johnson said in a Twitter post.

“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. Stay safe everyone, and please remember to stay at home to protect the NHS and save lives,” he said.

Downing Street said that it was considered sensible for doctors to see the UK PM in person given he has ongoing symptoms but he remains in charge of the government and in contact with ministerial colleagues and officials.

UK foreign secretary Dominic Raab, the first secretary of state, is the UK PM's chosen second in command and stepped in to chair the daily coronavirus response meeting, dubbed the COVID-19 war Cabinet, on Monday morning.

Johnson has until last Friday continued to chair the daily meetings remotely via video conference from his self-isolation in his private quarters in Downing Street since his diagnosis 10 days ago.

He was last seen in public applauding the NHS and other key workers just outside his flat in Downing Street last Thursday and posted his last Twitter video message on Friday in which he said he was still displaying minor symptoms.