LONDON — The British government hurtled into uncharted territory on Tuesday, with its foreign secretary, Dominic Raab, taking up the day-to-day duties of Prime Minister Boris Johnson, who was being treated in an intensive care unit as he battled a worsening case of the coronavirus.
Britain, with no written constitution, does not have a codified order of succession. That legal lacuna has prompted questions during prior episodes where prime ministers fell ill or underwent surgery, and now looms large at a time when Britain faces its greatest crisis since World War II.
Mr. Raab, 46, as first secretary of state, would become the government’s de facto leader government if the prime minister cannot carry out his duties. He was “deputized” by Mr. Johnson on Monday, chaired government meetings about the pandemic on Monday and Tuesday, and will probably take on additional duties, with Mr. Johnson’s prognosis so uncertain.
The government said Mr. Johnson, who has suffered symptoms of the virus for 11 days, was moved into intensive care on Monday evening after his condition deteriorated sharply. He has received “standard oxygen treatment” but is breathing on his own and has not been put on a ventilator, officials said on Tuesday. Nor has he been diagnosed with pneumonia, they said.
How the government will function if the prime minister is out for a prolonged period, or dies, is not yet clear.
“If he is incapacitated for quite a long time, then you’re in totally different circumstances,” said Jonathan Powell, who was chief of staff to Prime Minister Tony Blair. “In the middle of a crisis like this, in the modern world, it is quite impossible to function without a prime minister.”
The government will face momentous decisions, including when and how to lift the lockdown on Britain. Mr. Johnson, 55, had been leading that process and communicated the government’s measures to the public in daily briefings, where his familiar shambling style gave way to a graver mien.
Mr. Raab, by contrast, has been a peripheral figure in the government’s response, mostly focusing on organizing evacuation flights to bring back Britons stranded overseas. He is best known for his hard-line views on Brexit, which helped him get his post in Mr. Johnson’s pro-Brexit cabinet.
Mr. Raab, officials said, was working from his office in the Foreign Office. He would chair meetings on national security, though in the case of a security emergency, the cabinet would make decisions on a collective basis.
While he is trusted by the prime minister, Mr. Raab is only one of several ambitious ministers who may assert their right to step up. Michael Gove, a senior cabinet minister and longtime rival of Mr. Johnson, was the face of the government on television Tuesday morning, though he announced that he, too, was now in isolation at home after a member of his family showed mild coronavirus symptoms.
Other ministers — like the chancellor of the Exchequer, Rishi Sunak, and the health secretary, Matt Hancock — have been leading key parts of the response and will have strong voices. Keeping discipline could be difficult, analysts said, with much of the responsibility for that falling to the cabinet secretary, Mark Sedwill.
If Mr. Raab were to become incapacitated, officials said, Mr. Sunak would be next in line as de facto leader. As chancellor, he has gotten good reviews for rolling out gargantuan packages to rescue an economy under lockdown.
In one sense, Mr. Raab’s less direct involvement could be an advantage, allowing him to resolve potential disputes between economic and health officials over the easing of social distancing measures.
The son of a Czech refugee who fled the Nazis in 1938, Mr. Raab is not known for his diplomatic skills. He served as Brexit secretary under Mr. Johnson’s predecessor, Theresa May, but quit her government after clashes over her negotiations with the European Union.
In 2012, he was a contributor to “Britannia Unchained,” a book that laid out a vision of a free-market, deregulatory future for a post-Brexit Britain. He mounted an unsuccessful run for party leader last summer.
If Mr. Johnson died or were permanently sidelined, said Mujtaba Rahman, an analyst at Eurasia Group, the cabinet would probably agree on a caretaker prime minister — probably Mr. Raab — and schedule a vote for the Conservative Party to choose a new leader.
But holding a vote would be difficult until the worst of the pandemic passed, and a full-blown campaign, like the one that ended in Mr. Johnson’s election as party leader last July, seems out of the question for now.
Mr. Johnson is only one of several prominent officials to be struck by the virus. Mr. Hancock, the heath secretary, had symptoms and emerged from self-isolation late last week. The chief medical officer, Chris Whitty, made his first public appearance on Monday after a week of self-quarantine.
Mr. Johnson’s most influential adviser, Dominic Cummings, was filmed hurrying out of Downing Street the day after his boss tested positive for the virus. He has not been seen in public since.
No other Western government has been so ravaged by the virus, and it comes after three and half years of political upheaval. Even before this crisis, analysts said Mr. Johnson’s cabinet was weak, in part because he purged several senior party members during last fall’s bitter debate over Brexit.
Downing Street has released few details about Mr. Johnson’s condition. On Monday, Mr. Raab told reporters that the prime minister had spent a comfortable night and was in good spirits. But he said he had not spoken to Mr. Johnson since Saturday. Two hours after that soothing prognosis, Mr. Johnson was being moved into intensive care.
British governments have typically tried to hush up cases where prime ministers had health problems. In 1953, Winston Churchill suffered a serious stroke and was spirited to his country house, Chartwell, where he recuperated in secrecy with a nurse and his private secretary bringing him government papers.
For weeks, his secretary, Jock Colville, later wrote, “My colleagues and I had to handle requests for decisions from ministers and government departments who were entirely ignorant of the prime minister’s incapacity.”
In 1983, Margaret Thatcher had an eye operation for a detached retina at a private clinic in Windsor. The government did not disclose her location, but the press discovered it and the surgeon later spoke to reporters.
In 2003 and 2004, Mr. Blair underwent two heart procedures for cardiac arrhythmia, at least one of which required sedation. In both cases, Mr. Powell recalled, “We kept it hush-hush and didn’t brief it beforehand.” But Mr. Blair was back on the job within 24 hours and suffered no lingering effects.
Part of the problem, Mr. Powell said, was that the role of prime minister has evolved from being first among equals in an empowered cabinet to more of a singular national leader, in the mold of an American or French president.
“We don’t have a cabinet system in the old way,” Mr. Powell said. “Since Thatcher, the prime minister has been more of a president. We need a prime minister in place to make the key decisions.”