"Over the course of this afternoon, the condition of the Prime Minister has worsened and, on the advice of his medical team, he has been moved to the Intensive Care Unit at the hospital.
The spokesman went on to say the PM is alert and “receiving excelling care” but has requested Foreign Secretary Dominic Raab step up and deputise for him in his absence.
Last month, the PM’s spokesman reportedly confirmed that if Mr Johnson became unwell and was unable to work, Mr Raab would stand in.
Following the news of Mr Johnson’s admission, Mr Raab reportedly told the BBC there is an “incredibly strong team spirt” behind the PM, who are making sure his plans are actioned.
"That's the way we'll bring the whole country through the coronavirus challenge," Mr Raab said.
The Prime Minister was first admitted to hospital 10 days ago, after he developed symptoms, including a temperature and cough, and tested positive for coronavirus.
Despite his admission overnight, the PM took to Twitter on Monday to give an update on his condition, telling people he was in "good spirits".
“Last night, on the advice of my doctor, I went into hospital for some routine tests as I’m still experiencing coronavirus symptoms,” Mr Johnson began.
“I’m in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe,” he added.
The PM later wrote: “I’d like to say thank you to all the brilliant NHS staff taking care of me and others in this difficult time. You are the best of Britain.
“Stay safe everyone, and please remember to stay at home to protect the NHS and save lives,” he added.