Foreign Secretary Dominic Raab, who is carrying out the prime minister's duties in his absence, said Johnson was "receiving standard oxygen treatment and breathing without any assistance. He's not required any mechanical ventilation."
Covid-19, the disease caused by the novel coronavirus, can inflame the airways and make it difficult to breathe. Some patients go on to develop pneumonia, which can lead to acute respiratory distress syndrome, an often fatal outcome.
Johnson, 55, has no known underlying health conditions. His official spokesman said the prime minister was not suffering from pneumonia, but persistent cough and fever. The spokesman said, "The prime minister's condition is stable and he remains in intensive care for close monitoring. He is in good spirits."
Cabinet Office Minister Michael Gove said Tuesday that the government "will ensure the country is updated" on Johnson's status. Before the transfer to the ICU at St. Thomas' Hospital in London on Monday evening, government officials evaded questions about his condition and what tests and treatment he was receiving.
Later on Tuesday, Gove said he himself would have to go into isolation at home for 14 days, after a family member began to display mild symptoms of coronavirus infection.
Asked at a news conference Tuesday evening who would make tough decisions with Johnson in ICU, Raab downplayed his role, stressing that "normal Cabinet collective responsibility" would apply.
The foreign secretary said there was "total unity" and "total resolve" among cabinet members, who had been given "very clear instructions" by the prime minister to continue with their plan to fight the pandemic.
He suggested the country's lockdown would not be relaxed next week, when the government has scheduled its first formal review of the effectiveness of its stay-at-home order.
Britain has confirmed 55,242 coronavirus cases and reported 6,159 deaths. The country, like the United States, expects those numbers to continue to surge for days and maybe weeks.
Johnson's illness has stunned Britain, sparking an outpouring of support for the prime minister and fear of what might come next.
Headlines of newspaper tabloids read: "Sick Boris Fights for Life" and "Still Conscious and Battling."
Queen Elizabeth II sent a message on Tuesday to his family and fiancee, Carrie Symonds, who has been self-isolating with coronavirus symptoms and is pregnant with the couple's first child. "Her Majesty said they were in her thoughts and that she wished the Prime Minister a full and speedy recovery," according to a palace statement.
"I'm confident he'll pull through," Raab said, "because if there's one thing I know about this prime minister, he's a fighter."
Raab is not a well-known politician in Britain. He lost a Conservative Party leadership contest to Johnson last summer, and he lacks the prime minister's gift of elocution, his humor and his bombast. Raab, who has a black belt in karate and law degrees from Oxford and Cambridge, briefly served as Brexit secretary under Johnson's predecessor, Theresa May. He was roundly mocked for saying he hadn't quite understood" how reliant Britain's trade was on the Dover-Calais crossing.
Johnson became one of the first world leaders to test positive for the coronavirus, on the night of March 26. He was admitted to the hospital on Sunday evening. Until he was moved to the ICU, his staff maintained that he was leading the country from his bed.
"His general air of vigorousness means that many people are shocked that he should be struck down," said Andrew Gimson, author of "The Adventures of Boris Johnson."
Gimson said Johnson was "one of the very few British politicians who can enter a shopping center or mall on a rainy Wednesday afternoon when nothing else is going on and completely change the atmosphere. He has an ability to connect with people."
He said Johnson "has always regarded illness as something you work through, rather than taking to your boudoir and lying in the dark until it all passes, you carry on in a tough energetic way."
That attitude may have kept Johnson from resting more during the early days of his illness.
It is also in keeping with the outlook of his idol, Winston Churchill. In December 1941, during a trip to Washington, Churchill had a minor heart attack. His doctor told him he was just overworking. Churchill continued on his way to Ottawa to address the Canadian parliament.