A top GP has warned that Boris Johnson's move to an intensive care unit means the prime minister is "very unwell".
Johnson's condition worsened on Monday after he tested positive for coronavirus 11 days ago.
Number 10 said on Sunday he had been moved to St Thomas' Hospital in London as a precautionary measure for "routine tests".
The 55-year-old was move to intensive care at around 7pm on Monday evening after his condition worsened.
GP Ellie Cannon said the move will have been because he is so ill that medics looking after him deemed the move essential.
Speaking about his condition on Sky News, she said it was "very serious".
A number of intensive care experts speaking to the media have said the prime minister's recovery will not be straightforward at all.
Dr Cannon told Sky News: "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."
The Daily Record reported that the Dr 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 Johnson.
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 intubated.
"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."
Confirming the PM's move to intensive care, a Number 10 spokesperson said: 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."
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