Foreign Secretary Dominic Raab has been designated to take over while the prime minister is in hospital, but the formal rules are unclear.
Senior ministers arrived at 10 Downing Street on Tuesday morning to hold their first cabinet meeting without UK Prime Minister Boris Johnson, who spent spent last night in intensive care suffering from coronavirus.
Foreign Secretary Dominic Raab was due to chair the meeting, having been designated to take over while Johnson is incapacitated.
But the UK currently has no official post of deputy prime minister, and the prime minister's absence has raised questions concerning how the government is run, and how key decisions will be made.
With the country still yet to reach the peak of the coronavirus outbreak, the British government is under intense pressure. More than 50,000 people have been confirmed to have contracted the virus, and more than 5,000 have died.
Who is Dominic Raab and is he really in charge?
Dominic Raab, 46, has the title "first secretary of state" and is a former lawyer who was first elected to parliament in 2010 and entered government five years later.
He came to national prominence as an influential Brexit supporter, and in 2018 was appointed as minister for leaving the European Union under Theresa May's premiership.
Boris Johnson promoted him to foreign secretary after taking over as prime minister in the wake of last year's Conservative Party leadership contest. Recently, Raab oversaw the return of thousands of Britons stranded abroad as borders closed and flights were cancelled amid the coronavirus pandemic.
"The government's business will continue," Raab said late on Monday. He said Johnson had asked him "to deputise for him where needed in driving forward the government's plans to defeat coronavirus".
But it is not clear whether the foreign secretary -- who is is relatively inexperienced at top government level -- can command the same authority as Johnson.
Tensions have been aired in public between ministers and civil servants -- and there are reportedly strains within the cabinet too, with the potential for a clash between health and economic priorities.
Another significant factor is that several government members and support staff are out of action or self-isolating amid the pandemic.
How is the prime minister's replacement chosen?
The country has no written constitution and there is no clear line of succession for when the leader is taken out of action -- unlike in the United States where power is handed firstly to the vice president, and then to other designated figures in order.
Even when the UK has had a deputy prime minister, this has not always meant the person occupying the post would automatically take over.
The British constitution is unwritten and uncodified, and arrangements tend to be improvised and evolve as events unfold. Should Dominic Raab become incapacitated, the UK's Chancellor (finance minister) Rishi Sunak would take over, Downing Street has said.
Under the royal prerogative the head of state (the Queen) chooses the successor to a prime minister, but the political reality is that circumstances determine the new leader to emerge.
The UK Institute for Government says British prime ministers remain in office until they resign or die.
Although in circumstances such as today's, an "acting leader" can be appointed, if a more permanent successor to the prime minister had to be chosen, it would be up to the cabinet to make a recommendation to the Queen.
What happened when previous prime ministers became incapacitated?
Seven British prime ministers have died in office, in which case the tradition is that the monarch asks another member of the government to take over.
In both world wars during the last century, wartime UK leaders were taken ill -- but their conditions were kept hidden from the public.
Weeks before the end of World War I, Prime Minister David Lloyd George contracted "Spanish flu" as the pandemic took hold. He was confined to bed but news of his condition was not released as it was thought that this would sap public morale.
Likewise, in the Second World War, reports have said Winston Churchill suffered a mild heart attack at the White House while on a trip to the US in December 1941. It did not deter him from visiting Canada and returning to Washington during the following days.
Other prime ministers have died from serious illness soon after leaving office, notably Andrew Bonar Law in 1923 and Neville Chamberlain in 1940.