While deputy prime ministers or members of the cabinet have chaired government meetings in the past, Dr Andrew Blick said a question remained over who would have the final say if a contentious decision needed to be made.

Speaking to RTE 1 Radio in the Republic of Ireland, Dr Blick said:  “This kind of thing doesn’t come along very often and it’s complicated a bit further in the UK in that we don’t really have specific arrangements for how to handle this kind of instance, where the prime minister is incapacitated and is unavailable.

“Perhaps in other systems there might be a specific deputy prime minister or vice president or equivalent but we don’t have that. We sometimes have people who are called deputy prime minister but exactly what that role is can vary greatly and there is no actual requirement to have one.

“So it is an unusual circumstance in that we don’t have a clear pre-arranged way of dealing with this.”

It is understood that the First Secretary of State, Dominic Raab, was nominated by the Prime Minister Boris Johnson to act ‘as necessary’ in his absence but the extent of his ability to do so remains unclear.

Mr Johnson was taken into intensive care at a London hospital on Monday (6 March) and remains unable to lead the government while he receives treatment for Covid-19.

Dr Blick, director of the Centre for British Politics and Government, said: “My understanding is that [Mr Raab] is taking over as necessary but he has not become the prime minister and he is not acting prime minister. Historically, though, it’s not that unusual to have someone else chairing cabinet meetings.

“During the Second World War, for instance, when Churchill was away on diplomatic matters, Clement Atlee, the deputy prime minister, often chaired cabinet meetings so this can go on and there’s nothing constitutionally extraordinary about that.

“The cabinet can function collectively, we do have a principle of collective government, but ultimately, there is a reason we have a prime minister and there may be decision that needs to be made or somebody may need to take a lead and the question arises ‘who is the person who does that?’.”

Dr Blick added: “Decisions will have to be made. So far they’ve been able to proceed by consensus but if some kind of decision point comes up further down the line. At some point decisions may need to be taken and it may be that consensus isn’t as easy to reach over it. It may be that somebody needs to come forward.

“That’s where the job of the prime minister comes in. Decisions are taken collectively but the prime minister can sometimes set the agenda or push things in a particular direction. Who is going to do that if that time comes? We don’t know yet. With a bit of luck, maybe the prime minister will be back in action again by that point.”