Government officials in London insist the move was merely "precautionary". But the revelation that he needed oxygen to help him breathe - though not through a ventilator - before he was moved, while still conscious, to the intensive care unit at St Thomas Hospital in central London, just opposite Big Ben and the Houses of Parliament, indicates that his condition is worse than initially admitted.
Mr Johnson, who turns 56 in June, has had no previous medical complaints. He launched his political career two decades ago by riding a bicycle, and has long been Britain's most famous cyclist-politician.
He is also a keen tennis player, and although the job of prime minister has severely limited the time he has to engage in any sport, Mr Johnson has been following online yoga classes.
The British Prime Minister has struggled, however, to keep his weight down; he apparently weighs over 100kg which, at a height of 1.75m, puts him in the seriously overweight category.
Obesity is a known vulnerability for those exposed to the coronavirus. British statistics indicate that more than 70 per cent of those admitted to intensive hospital care as a result of a coronavirus infection are classified as obese, although it is also true that two-thirds of the country's overall population is classified as overweight.
Still, what is clear is that officials in London have persistently played down the true state of their Prime Minister's health condition.
When Mr Johnson was first placed under quarantine at his 10 Downing Street official residence on March 26, communiques suggested that he was feeling fine and was handling all government business as normal, while his pregnant 32-year-old fiancee Carrie Symonds was showing symptoms of the virus, though she had not been tested.
Senior British journalists soon picked up rumours that the Prime Minister's true condition was worse than officially admitted, and Mr Johnson did not look at all well when he made a brief public appearance outside his residence at the end of last week to join the nation in applauding Britain's health workers.
Editors at newspapers and other mainstream media outlets decided to observe silence on the matter, in a rare show of respect for the privacy of a public figure, not the sort of behaviour the British media is otherwise famous for.
But the debate about the Prime Minister's condition became public on Monday morning, when it was revealed that Foreign Minister Dominic Raab, who was deputised to coordinate Cabinet activities for the Prime Minister, admitted that he had not actually seen Mr Johnson for days.
The episode confirms a culture of secrecy which often hides the true health condition of British prime ministers.
Winston Churchill, for instance, suffered a heart attack in 1953 and was incapacitated for more than a month, yet the official line from London was that the prime minister was merely suffering from a cold.
Decades later, the British public was also kept in the dark about the fact that then Prime Minister Harold Wilson was diagnosed with Alzheimer's disease not long after his 1974 election.
The chief explanation for this culture of secrecy is that every prime minister feared that the slightest admission of a health problem would immediately unleash a race for his or her succession.
Secrecy was, therefore, their defence. Three out of the 25 prime ministers who ruled Britain over the past 120 years died within a few weeks or months after being compelled to leave office because they were terminally ill.
But not one of them publicly owned up to their grave health condition while in office, and one - Henry Campbell-Bannerman, in April 1908 - actually died in the official Downing Street residence as he was about to collect his belongings after his resignation for health reasons.
For the moment, any speculation about the need to even consider a replacement to Mr Johnson is deemed too distasteful in both government and opposition circles.
"Terribly sad news," tweeted Mr Keir Starmer, the newly elected leader of Labour, Britain's main opposition party. "All the country's thoughts are with the prime minister and his family," he added.
But government ministers have already given up pretending that Mr Johnson is still in effective charge of the country; as Foreign Secretary Dominic Raab now delicately puts it, British ministers are "making sure that we deliver all of the plans the Prime Minister wanted us to deliver as soon as possible".
The rules of political succession remain opaque. While Mr Raab, the Brexit minister under Mr Johnson's predecessor, Mrs Theresa May, is the person entrusted by the Prime Minister with the continuity of government, it will be up to the Cabinet as a whole to decide who should be recommended to Queen Elizabeth II for appointment as a caretaker prime minister, should the need arise.
The 93-year-old monarch, now kept at a safe distance from the pandemic, is apparently being briefed on an hourly basis.
The pound fell after news of Mr Johnson's worsening condition broke but steadied on Tuesday.
Mr Johnson’s spokesman said on Tuesday that the Prime Minister’s condition was stable and he remained in good spirits.
World leaders have conveyed their good wishes to the British leader, including United States President Donald Trump, who sent his "best wishes to a very good friend of mine".
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