Mr Johnson was moved to the intensive care unit of St Thomas’ Hospital in London early last night after his COVID-19 symptons worsened.
The PM was given oxygen on Monday afternoon and moved to intensive care as a precaution so he can be close to a ventilator.
Now a Facebook campaign has been set up urging people to come to their doorsteps, windows and balconies at 8pm tonight for a nationwide round of applause and show of support for the PM who has handed temporary control to Foreign Secretary Dominic Raab.
Downing Street said Mr Johnson, 55, was moved to the unit on the advice of his medical team and was receiving "excellent care".
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He was admitted to hospital with "persistent symptoms" on Sunday evening.
World leaders - including US President Donald Trump and French President Emmanuel Macron - have expressed their support for Mr Johnson.
A No 10 statement read: "The prime minister has been under the care of doctors at St Thomas' Hospital, in London, after being admitted with persistent symptoms of coronavirus.
"Over the course of [Monday] 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."
It continued: "The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication."
Mr Raab - who will later chair the government's daily Covid-19 meeting - said there was an "incredibly strong team spirit" behind the prime minister.
He added that he and his colleagues were making sure they implemented plans Mr Johnson had instructed them to deliver "as soon as possible".
"That's the way we'll bring the whole country through the coronavirus challenge," he said.
Newly elected Labour leader Sir Keir Starmer described it as "terribly sad news".
"All the country's thoughts are with the prime minister and his family during this incredibly difficult time," he added.
Meanwhile, Mr Trump said Americans "are all praying for his recovery".