Coronavirus Live Updates: Trump Attacks W.H.O. and Ousts Watchdog for Pandemic Fund

Mr. Johnson was transferred to the intensive care unit on Monday after his illness worsened. Aides said he had been moved in case he needed a ventilator to help his recovery. On Tuesday evening, the British foreign secretary, Dominic Raab, said that Mr. Johnson was “receiving standard oxygen treatment and breathing without any assistance,” like a ventilator.

As Britain has no written Constitution and no standard line of succession in the case of illness or death of the head of the government, it was for Mr. Johnson to decide who should stand in for him if he became ill. But the man he nominated, Mr. Raab, has been relatively untested, serving as the leader of the Foreign Office for less than a year.

While Mr. Johnson remains as the head of the government from his hospital bed, the seriousness of his illness means that could change quickly. At a time of extraordinary challenge, Mr. Raab is already serving as chairman of a key committee on the pandemic as the government battles to control the spread of the virus and stabilize an economy hit hard by the lockdown measures it has imposed.

Previous British prime ministers, including Tony Blair and Margaret Thatcher, have had health issues while in power, but had brief periods of absence for planned procedures.

Mr. Johnson could be hospitalized for some time, and at a moment when the government must make major decisions about its virus response. Though some British prime ministers have nominated deputies, Mr. Johnson chose not to do so when he took the role last year.

The last time Britain experienced such a power vacuum was in 1953, when Winston Churchill suffered a stroke and the truth of his condition was kept from the British public.

Before going into intensive care, Mr. Johnson asked Mr. Raab to stand in for him “where necessary.”

Another senior minister, Michael Gove — who has had a lead role in coordinating the government’s response, including giving interviews on Mr. Johnson’s state of health — announced on Twitter on Tuesday that he was self-isolating. He felt well, he said, but a member of his family showed symptoms of the virus.