British Prime Minister Boris Johnson was moved to an intensive care unit on Monday after his coronavirus symptoms worsened, but 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.
"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. "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 urged caution about media reports that Johnson was receiving oxygen treatment, but refused to answer directly after being asked whether he had pneumonia.
The Times newspaper and ITV correspondent Robert Peston said Johnson had received oxygen.
Raab, 46, chaired the government's emergency COVID-19 meeting on Monday.