Boris Johnson "almost took one for the team" and will need a period of rest as he begins to recover from Covid-19, his father has said.
Stanley Johnson said the whole family is “amazingly grateful” for the efforts of the NHS and for the huge outpouring of support for his son.
And he played down suggestions that the prime minister will quickly return to work at Number 10, saying he "has to take time".
Mr Johnson Senior’s comments are likely to be taken as a further indication that Foreign Secretary Dominic Raab will continue to deputise for the PM in the coming days.
The Prime Minister has returned to a ward at St Thomas’ Hospital in London after spending three nights in intensive care being treated for symptoms of the coronavirus.
In an interview with BBC Radio 4’s Today programme, Mr Johnson Snr said his son’s illness had underlined the seriousness of the pandemic.
"To use that American expression, he almost took one for the team. We have got to make sure we play the game properly now,” he said.
"This is pretty straightforward now," he went on.
"He must rest up. As I understand it, he has moved from the ICU into a recovery unit but I don’t think you can say this is out of the woods now."
He added: "He has to take time. I cannot believe you can walk away from this and get straight back to Downing Street and pick up the reins without a period of readjustment."
The PM was admitted to hospital on Sunday evening for tests due to suffering continuing symptoms of Covid-19, having tested positive for the virus 10 days earlier.
He now appears to be on the road to recovery, with Downing Street saying on Thursday evening that he has left intensive care and is being closely monitored.
As well as his position as Foreign Secretary, Mr Raab is First Secretary of State and it was in this role that the prime minister asked him to stand in for him after his symptoms worsened on Monday afternoon.
In addition to leading the Government’s daily coronavirus “war cabinet” sessions, Mr Raab will also chair any necessary meetings of the National Security Council in Mr Johnson’s stead, it has previously been confirmed.
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