But, having been in hospital for tests and observation, it emerged on Monday night that his doctors advised he be admitted to intensive care.
MPs from across the political spectrum, world leaders and other official figures have spoken out to send Mr Johnson and his family messages of support after his condition deteriorated.
Chancellor Rishi Sunak tweeted: “My thoughts tonight are with Boris Johnson and Carrie Symonds.
“I know he’ll be getting the best care possible and will come out of this even stronger.”
Home Secretery Priti Patel tweeted a picture of herself with the PM. She added: “My love & thoughts are with Boris Johnson, Carrie Symonds & the Prime Minister’s family. for you & thinking of you.”
Health Secretary Matt Hancock tweeted: “Sending all possible best wishes to Boris Johnson and his loved ones. I know he will receive the best possible care from our amazing NHS.”
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.”
Canadian Prime Minister Justin Trudeau tweeted: “Sending my best wishes to Prime Minister BorisJohnson for a full and speedy recovery. My thoughts are with you and your family right now. Hope to see you back at Number 10 soon.”
Pedro Sanchez, the Prime Minister of Spain, which has also suffered a high number of cases of coronavirus, tweeted: “My solidarity and wishes for a speedy recovery for the Prime Minister BorisJohnson.
“These are difficult days for our countries, but from strength and unity, we will be able to win this battle. A hug to all the British people.”
Mayor of London Sadiq Khan tweeted: “Praying for the Prime Minister’s swift recovery tonight.
“[Guy’s and St Thomas’] has some of the finest medical staff in the world, and he couldn’t be in safer hands.”
Labour leader Sir Keir Starmer tweeted: “Terribly sad news. All the country’s thoughts are with the Prime Minister and his family during this incredibly difficult time.”
Members of the newly-formed shadow cabinet also offered their support for Mr Johnson.
Shadow justice secretary David Lammy tweeted: “Awful news. My very best wishes to the Prime Minister, as well as his partner Carrie, family and friends. Get well soon Boris Johnson.”
Labour deputy leader Angela Rayner posted: “Sending my best wishes at this worrying time to the PM BorisJohnson and Carrie Symonds and hoping that his condition stabilises in his battle with coronavirus.”
And shadow deputy mental health minister Dr Rosena Allin-Khan tweeted: “I wish the Prime Minister a full and speedy recovery and thank the NHS team looking after him and everyone at St Thomas’.
“Coronavirus can affect anyone, the symptoms can be really awful. Please, please, remind your friends and family to stay at home!”
Mr Johnson’s predecessor Theresa May tweeted: “My thoughts and prayers are with BorisJohnson and his family as he continues to receive treatment in hospital. This horrific virus does not discriminate. Anyone can get it. Anyone can spread it. Please #StayHomeSaveLives.”
Former Conservative Prime Minister David Cameron also sent a message of support to Mr Johnson.
He said: “Thinking of Boris Johnson and his family tonight. Get well soon. You are in great hands and we all want you safe, wekk and back in 10 Downing Street.”
Archbishop of Canterbury Justin Welby tweeted: “The news that our Prime Minister has been moved to intensive care deepens our compassion for all who are seriously ill and for those caring for them. I invite all people of faith to join me in praying for BorisJohnson and his loved ones.”
Foreign Secretary Dominic Raab will “deputise where necessary” while he is in intensive care, a No 10 spokesman said.
The news came just a few hours after Mr Raab told a press briefing that the PM remained in charge of the Government despite remaining under observation.
Mr Raab later said there was an “incredibly strong team spirit behind” Mr 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.
The Conservative Party leader is understood to be conscious and has been moved to the ICU at about 7pm as a precaution should he require ventilation to aid his recovery.
Two uniformed police officers were stationed at the entrance to the hospital on Monday evening.
Throughout the day officers and police vehicles could be seen entering and exiting the central London hospital.