Foreign Secretary Dominic Raab is acting as deputy leader of the country 'where necessary" as the Prime Minister is treated in intensive care.
On Monday night, Downing Street announced Boris Johnson had been taken to intensive care after his condition worsened.
The Prime Minister has been in hospital since Sunday night with symptoms of coronavirus.
Two uniformed police officers were stationed at the entrance to St Thomas’ Hospital, where his treatment is taking place, on Monday evening.
Downing Street said Mr Raab would “deputise where necessary” while the Prime Minister was in intensive care.
A Number 10 spokesman said: "The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication.”
As the news broke on Sunday night, politicians and members of the public sent their best wishes and hopes for a swift recovery to the Prime Minister.
Chancellor Rishi Sunak tweeted: “My thoughts tonight are with @BorisJohnson and @carriesymonds. I know he’ll be getting the best care possible and will come out of this even stronger.”
Business minister Nadim Zahawi said: “Thoughts & prayers for @BorisJohnson & @carriesymonds and their family. I have known Boris for 20 years he is a fighter and will beat this virus.”
Newly-elected Labour leader Keir Starmer said: "Terribly sad news. All the country’s thoughts are with the Prime Minister and his family during this incredibly difficult time."
Sir Lindsay Hoyle, Speaker of the House of Commons, said: “This is terrible news. I know the thoughts and prayers of everyone across the House are with the Prime Minister and his family right now.
“We all wish him a speedy recovery.”
And Mayor of London Sadiq Khan tweeted: “Praying for the Prime Minister’s swift recovery tonight. "
"[St Thomas' Hospital] has some of the finest medical staff in the world, and he couldn’t be in safer hands.”
Brexit Party leader Nigel Farage posted: “I hope and pray for @BorisJohnson to come through this ordeal quickly.”
