A top GP has claimed Boris Johnson was moved to intensive care as a precaution - insisting it will be because he is gravely ill from coronavirus.
And the doctor warned that, should he recover from the virus, the Prime Minister may be left bearing emotional scars from the experience.
Johnson's condition worsened over the last 24 hours, after he had previously suffered from "mild symptoms" of coronavirus.
The 55-year-old was moved to the ICU in the St Thomas' Hospital, in London, at around 7pm on Monday, apparently as a precaution.
But Doctor Ellie Cannon, who runs an NHS surgery, said it is because he is so ill the NHS staff looking after him deemed the move essential.
Speaking on Sky News about Johnson's condition, she explained: "It's incredibly serious.
"I've spent the day today looking after patients in clinic who are at the milder end of the COVID-19 spectrum - people who are able to stay at home and to look after themselves.
"Going into intensive care - that is the serious type of care you can have.
"Intensive care is for the sickest patients, that is not done as a precautionary measure.
"He may have gone to hospital as a precautionary measure, but he will have gone in to intensive care because they're worried about the state of his health."
She scoffed at the suggestion he would have been admitted to the ICU because he is the Prime Minister, insisting it was "absolutely" because of his condition.
The doctor, who runs a surgery in North London, added: "We always make those decisions without fear or favour.
"It's very important that people understand that.
"I have patients who are so afraid when I tell them they might have Coronavirus because they're afraid they're going to go to hospital and they're going to die.
"That isn't the case for most people but yes, absolutely, if he needs intensive care treatment then he's obviously very unwell."
She said the ICU is where NHS staff believe is the best place for the PM - and gave advice on how to help battle the virus for those who have it.
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She added: "Intensive care, as I say, is the highest level of care.
"It's about very close monitoring, often there is one to one nursing, maybe two to one, obviously that's been affected at the moment by the staff shortages, highest level of monitoring from both nurses and from doctors and then there is organ support.
"People who need support for their lungs, we've spoken a lot about ventilation, when people are incubated.
"There's also all the other organ support as well - patients often need help with their heart function, with their kidney function.
"It's really, really intensive treatment.
"This really emphasises to everybody how important it is to sleep and to rest, to be very well hydrated, to drink much more than you would normally drink, and to really let your body recover and to fight off that virus."
She said he will need quite a while to recuperate, if he recovers from the virus and is discharged from hospital.
And she said the virus may scar the PM emotionally - as well as the NHS staff who are treating him.
She explained: "Intensive care is incredibly traumatic.
"We've seen reports today of how traumatic intensive care is for the staff working there.
"It's also incredibly traumatic for the patients who undergo that if they undergo ventilation, if they undergo incubation.
"He will absolutely need time to recover - both physically and mentally from that experience - especially, don't forget, as he's going through this totally publicly."
We told how politicians across the political divide tweeted messages of support shortly after he was admitted to the ICU.
A spokesperson for Number 10 said: “Since Sunday evening, 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 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 PM has asked Foreign Secretary Dominic Raab, who is the First Secretary of State, to deputise for him where necessary.
"The PM is receiving excellent care, and thanks all NHS staff for their hard work and dedication."