The PM has asked Foreign Secretary Dominic Raab to deputise "where necessary", No 10 says.
A look at the man in line to take over from Boris Johnson, if the PM's coronavirus gets worse.
The rules for what happens if a PM is incapacitated are unclear - but we have some ideas about how the situation may play out.
The former party leader is appointed shadow business secretary by Sir Keir Starmer.
Rochdale MP Tony Lloyd, 70, is said to be in a stable condition and responding to treatment.