“Boris Johnson himself put out a tweet on Monday saying he was in good spirits,” said Al Jazeera’s Nadim Baba, reporting from London.

“[Foreign Secretary] Dominic Raab at the daily briefing said much the same thing, insisting that Johnson, from hospital, continued to lead the government and receive regular updates – but he also admitted he hadn’t spoken to the prime minister since Saturday, and that raised a few questions and sceptical looks,” Baba added.

“We were told [when Johnson was admitted on Sunday] that he would receive routine checks – they could involve checking his lungs, but beyond that, there’s been very little detail of what treatment he has received and what condition he has been in. But now, it is clear that he is in a serious condition, and serious enough that he is not just going to be in hospital overnight, but for some while further.”

Reactions have been swift.

“Terribly sad news. All the country’s thoughts are with the prime minister and his family during this incredibly difficult time,” said Keir Starmer, leader of the main opposition Labour Party.

Rishi Sunak, the country’s finance minister, known as the chancellor of the exchequer, paid special mention to the prime minister’s heavily pregnant partner, Carrie Symonds, who is understood to be getting over the virus herself.

“My thoughts tonight are with Boris Johnson and Carrie Symonds. I know he’ll be getting the best care possible and will come out of this even stronger,” he said.

The news also shocked the markets, with the UK’s currency taking a major hit. Sterling turned negative against both the US dollar and the euro on Monday night.

In early February, as the UK took a different approach to much of the rest of Europe in its handling of the crisis, Johnson said he was continuing to shake hands with people – even in a hospital in which he said there were patients being treated for coronavirus infections.

Three weeks later, the country was placed in a state of near-lockdown, with movement restricted and all but “essential workers” advised not to leave their homes.