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 quarters in the Foreign Office. He would chair meetings on national security, though in the case of an 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 family member 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. On Tuesday, Mr. Raab signaled the government was not close to a decision on lifting the lockdown.