British Prime Minister Boris Johnson has been taken into intensive care in hospital after his COVID-19 worsened, his office said on Monday.

“Since Sunday evening, the Prime Minister has been under the care of doctors at St Thomas’ Hospital, in London, after being admitted with persistent symptoms of coronavirus,” Downing Street 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.

“The PM has asked foreign secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary.”

A source later said Johnson remained conscious.

Earlier, it was reported that Boris, who is 55, has been given oxygen. His spokesman had 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.

An Indian medical expert, Bharat Pankhania, senior clinical lecturer, University of Exeter Medical School, said the Prime Minister’s doctors would be playing it safe.

“If a patient is developing pneumonia, it can get progressively worse very quickly and hence early admission upon the first signs of difficulty with breathing are very important,” said Pankhania, one of many doctors who have come to the fore during the coronavirus crisis.

On Monday, a spokesman for the Prime Minister said Boris was not taken to St Thomas’s by ambulance: “The Prime Minister was admitted to hospital for tests as a precaution -– it was not an emergency admission. The issue is that his symptoms remain persistent.

“The Prime Minister had a comfortable night and is in good spirits. He remains in hospital under observation.”

Police and army presence around the hospital, overlooking the Thames river, was stepped up on Monday, while steel barriers were put up outside the entrance.

The news that Boris had been taken to the hospital five minutes from Downing Street emerged just as the Queen had finished addressing the nation and the Commonwealth at 8pm on Sunday.

The Queen, who has been holding her weekly audience with Boris not face to face but by telephone from Windsor Castle, said: “Across the Commonwealth and around the world, we have seen heart-warming stories of people coming together to help others, be it through delivering food parcels and medicines, checking on neighbours, or converting businesses to help the relief effort.”

She ended with a touch of Vera Lynn: “We should take comfort that while we may have more still to endure, better days will return: we will be with our friends again; we will be with our families again; we will meet again.”

Neither Prime Minister’s ministers nor his staff have been meeting Boris for the last 10 days.

Politicians from all parties have sent get-well messages to Boris. But President Donald Trump’s comment — “All Americans are praying for him” — was not seen as being exactly helpful.

The President did add: “He’s a great friend of mine, a great gentleman and a great leader. I’m sure he is going to be fine, he’s a strong man, a strong person….I just hope he’s OK.”