UK prime minister Boris Johnson, who is battling Covid-19, is in intensive care. Johnson, 55, was diagnosed with the disease 10 days ago, and he has largely been absent from public life ever since.

In the UK, health officials advise people to stay at home if they experience Covid-19 symptoms. The specialist care in hospitals is largely for people who are having difficulty breathing. The prime minister is in St. Thomas’, one of London’s biggest public hospitals.

Johnson’s hospitalization raises a grim question: Who in the UK government takes over if the prime minister becomes incapacitated, or even dies? Some countries have succession plans that are set in stone. The UK does, too, but it’s reserved for its monarch. If Johnson is no longer able to carry out his duties, it’s not clear what happens next. He is not the head of state in the UK, and his role is a matter of convention rather than constitution.

If a prime minister departs suddenly for any reason, the party to which they belong elects a new leader and, after confirmation by the Queen, that person becomes the new permanent prime minister. That process could take days, or even weeks.

In an emergency, like the one the UK is facing right now, the baton would most likely be passed first to one of the three most senior government ministers: the chancellor (finance minister), home secretary, or foreign secretary. But there is no unequivocal written rule that states which of those three would take charge.

In the event, Johnson asked Dominic Raab, the foreign secretary, to take over daily functions. Raab is also first secretary of state, which gives him seniority over the others, but the title is not always used and he does not have royal assent. There is no “interim prime minister.” The country is governed by the Cabinet, in any case, with the PM merely the first among equals.

It is also possible, constitutionally, to have no prime minister at all. Senior ministers could take turns running meetings, while the delivery of their policies has always been a matter for Britain’s civil service. Nature abhors a vacuum, however, and a crisis probably needs a leader.