Britain's constitution offers no clear answer to the question now on many Britons' minds: What happens if Prime Minister Boris Johnson, undergoing tests in hospital after persistent symptoms of coronavirus, cannot continue to lead.
Johnson was admitted to hospital on Sunday (Apr 5) in what his office said was a precautionary step after testing positive 10 days ago and still suffering from a high temperature. He remains in charge of the government, his office said.
Johnson has said he can keep working from self-isolation in his Downing Street residence, just as his health secretary, Matt Hancock, who also tested positive for the virus, has done.
But the fact that two such crucial leaders in the UK's fight against the pandemic have contracted the disease has raised questions about how the government would function without them at a time of global crisis.
The constitution - an unwieldy collection of sometimes ancient and contradictory precedents - offers no clear, formal Plan B or succession scenario, experts said.
We've not been in that kind of situation, we've not had to think about it from that point of view before, Catherine Haddon, a senior fellow at the Institute for Government, said soon after Johnson was first diagnosed.
Whereas in the United States the vice president steps up if the president dies or becomes incapacitated, Britain has no formal deputy or caretaker prime minister who would take over.
Downing Street has already said, however, that Foreign Secretary Dominic Raab would deputize if necessary.
Nor is there any guidance for such circumstances in the Cabinet Manual which sets out the rules and conventions for the running of government, and there is little precedence.
When asked about who would stand in for the prime minister, his spokesman said: The prime minister has the power to delegate responsibility to any of his ministers, but for now it is the prime minister and then the foreign secretary.
In June 1953, then-Prime Minister Winston Churchill suffered a stroke while in office. His illness was kept so secret that some senior ministers were unaware. Churchill surprised doctors by recovering to carry on his duties, returning to Downing Street and running the cabinet two months later.
More recently, Tony Blair twice underwent treatment for a heart condition while prime minister in the early 2000s, each time briefly cutting back on his workload for a couple of days. Officials said that if Blair were to have been incapacitated, his then-deputy John Prescott would have taken over until a new leader was elected.
There is no suggestion Johnson is unable to perform his job. Since his diagnosis, he has carried on leading the government's efforts through the use of teleconferencing.
Bob Kerslake, head of the civil service from January 2012 to September 2014, said Johnson's role was crucial at this time, stressing that visible leadership was essential.
Kerslake, speaking to Sky News last month after Johnson tested positive, said officials would need to know what would happen if senior ministers were unable to do their jobs.