UK Prime Minister Johnson in intensive care

British Prime Minister Boris Johnson remains hospitalized in an intensive care unit, and is receiving oxygen support -- the first leader of a major world power to suffer so drastically from coronavirus complications.

55-year-old Johnson was first admitted to London’s St Thomas’ hospital on Sunday night (April 5), after showing persistent symptoms, including a high temperature and a cough, for over 10 days.

Downing Street says that his condition deteriorated rapidly over 24 hours, although he has not been put on a ventilator as of Tuesday (April 7) morning.

Johnson’s personal battle with the virus comes as the country enters what scientists say is likely to be one of the most deadly weeks of the pandemic.

Over 5,300 people have so far died in Britain.

The UK doesn’t actually have a formal line of succession should a prime minister become incapacitated, but in the days prior Johnson had asked Foreign Secretary Dominic Raab to deputize for him "where necessary."

Even when Johnson was in hospital on Monday (April 6), Raab told a news conference that the PM was still in charge.

But now Raab stands at the helm -- starting with a COVID-19 emergency response meeting on Tuesday morning.

Also on Tuesday, it was reported that another member of Johnson's team, Cabinet Minister Michael Gove, is self isolating as a member of Gove's family is showing symptoms.