LONDON (Reuters) - What would happen if British Prime Minister Boris Johnson’s case of coronavirus - so far he has “mild symptoms” - were to become more serious or even incapacitate him and his team? Under Britain’s uncodified constitution, the answer is unclear.

Johnson and Health Secretary Matt Hancock both said they were able to keep working from self-isolation at home after confirming they had tested positive for the virus.

But the fact that two such crucial members of the British government have contracted the disease - and their top medical adviser is now self-isolating with symptoms - has raised questions about how the government would function without them at a time of global crisis.

With only an unwieldy collection of sometimes ancient and contradictory precedents to go by, there is no simple, formally-enshrined “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, told Reuters.

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 deputise 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.”

CHURCHILL’S STROKE

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.

Britain's Prime Minister Boris Johnson leaves Downing Street, as the spread of coronavirus disease (COVID-19) continues. London, Britain, March 25, 2020. REUTERS/Hannah Mckay

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 had he been incapacitated, his then-deputy John Prescott would take over until a new leader was elected.

There is no suggestion Johnson is unable to perform his job, and his spokesman said he could carry on as before, although he was now doing so via teleconferencing.

Bob Kerslake, head of the civil service from January 2012 to September 2014, agreed that Johnson and ministers could continue to operate by video, but said there were potential drawbacks.

“It is a cabinet government but the prime minister’s role is crucial at this time, not least ... because of the visible leadership that the country needs,” he told Sky News.

MUDDLE THROUGH?

Kerslake said officials would need to know there was a system for what would happen if senior ministers were unable to do their jobs.

Losing Cabinet Office minister Michael Gove, who coordinates policy across government, would be a serious blow.

“He is critical to all of this,” Kerslake said. “If, for whatever reason, he was ill, who takes over from him?”

Haddon said some powers were specifically vested in cabinet ministers, so there was an issue of what happened if they were unavailable.

“If you got to a stage where ... you had secretaries of state who aren’t able to perform their functions, then there are question marks about whether junior ministers in their department act on their behalf,” she said.

One lawmaker in Johnson’s party, who has repeatedly tried to bring in a law to formalize who would replace a prime minister in the event of incapacity, said last week no one seemed to know what would happen.

“In a national emergency, you don’t want to be scrabbling around worrying about who’s in charge,” Peter Bone told the Mirror newspaper.

However, Haddon said naming Raab as Johnson’s substitute would prevent a political squabble among senior ministers over who fronted press conferences or chaired meetings.

“It is valuable for them to work out contingencies for various scenarios and they have obviously done a certain amount of thinking about that,” she said.

She said prime ministers and cabinet ministers were often absent and government operated in their absence.

“Secretaries of state go on holiday and their department functions without them. The prime minister goes on holiday and the rest of government is able to continue working,” she added.

“If there are things that (are) invested in a secretary of state and it is not proper for someone to act on their behalf, that’s when it becomes a problem.”