"The PM has asked Foreign Secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary".
Housing and communities minister Robert Jenrick told BBC Breakfast on Monday morning: "He spent the night in hospital and of course we all wish him well and we hope that as a result of these tests he will be able to come back to Downing Street as soon as possible". Those tests are under way and he will stay in hospital as long as he needs to.
Earlier in the day, US President Donald Trump during White House press briefing extended his well wishes. "He's a friend of mine, he's a great gentleman and a great leader".
His pregnant partner, Carrie Symonds, said she has been ill with symptoms for a week but was now recovering.
Concerns had been growing about Johnson's welfare ever since he posted a message Friday saying that he was feeling better, though was still feverish. The government's chief medical adviser, Prof Chris Whitty, has also had to self-isolate after showing symptoms.
The news of Johnson's hospitalisation comes as countries across the globe struggle to contain the virus.