The Prime Minister remains Prime Minister until he resigns or is otherwise unavailable. Foreign Secretary Dominic Raab is the designated survivor.
Boris Johnson has no formal deputy. But as Covid-19 hit, it was briefed that his “designated survivor” is Raab.
It was Raab who chaired the emergency Covid meeting on Monday morning and Downing Street said he would deputise “where necessary” while the PM is in hospital.
There is actually no official constitutional post of deputy prime minister, although the title has been given to political allies of Prime Ministers from time to time.
For now Raab takes over the fight against the virus and will also take security decisions as necessary.
However, moving from his position as First Secretary of State to Prime Minister would be widely seen a stopgap while Johnson recovers.
If the worst happens Britain’s cabinet government means that ministers would choose a Prime Minister among their ranks to carry on until a new leader of the Conservative Party could be chosen.
Mr Raab said there was an "incredibly strong team spirit behind" Boris Johnson and that ministers were determined to carry out his "direction".
"The Government's business will continue," he said.
"The Prime Minister is in safe hands with that brilliant team at St Thomas' hospital, and the focus of the Government will continue to be on making sure that the Prime Minister's direction, all the plans for making sure that we can defeat coronavirus and can pull the country through this challenge, will be taken forward."
He added: "There's an incredibly strong team spirit behind the Prime Minister, and making sure that we get all of the plans the Prime Minister's instructed us to deliver, to get them implemented as soon as possible.
"And that's the way it will bring the whole country through the coronavirus challenge that we face right now."
