UK Prime Minister Boris Johnson, who was already in hospital being treated for COVID-19, has been urgently moved to the ICU.

Downing Street has revealed the 55-year-old British Prime Minister was given oxygen on Monday afternoon after his respiratory symptoms got worse and was moved to an intensive care unit in London a short time later.

In a pre-emptive move, Mr Johnson asked his Foreign Secretary Dominic Raab to deputise “where necessary”.

It comes after just 24 hours after Mr Johnson was admitted to hospital with “persistent symptoms” linked to his positive COVID-19 diagnosis last week.

Mr Johnson tweeted his thanks to the health workers “taking care of me and others in this difficult time”.

Mr Raab said he and his colleagues were acting “as soon as possible” on plans Mr Johnson had instructed them to deliver.

In a public statement by Number 10 Downing Street, a spokesperson confirmed the PM was “receiving excellent care”.

“The prime minister has been under the care of doctors at St Thomas’ Hospital, in London, after being admitted with persistent symptoms of coronavirus,” it read.

“Over the course of the 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. The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication.”

The news was met with an outpouring of support from leaders across the world, including Scott Morrison who wished Mr Johnson his best and to “get well soon”.

Labour leader Keir Starmer said it was “terribly sad news”, wishing the PM and his family the best at such an “incredibly difficult time”.

Mr Johnson is one of more than 51,600 Britons diagnosed with coronavirus as of yesterday, with the death toll continuing to climb to 5,373 so far.

More Coronavirus updates.