Boris Johnson was moved to the critical unit at St Thomas' Hospital on Monday evening as part of his ongoing treatment for Covid-19.
He was understood to be conscious when he was moved to intensive care at about 7pm, as a precaution should he require ventilation to aid his recovery.
Derek Hill, professor of medical imaging at University College London (UCL), said the PM could be given a breathing aid known as continuous positive airway pressure (CPAP), which bridges the gap between an oxygen mask and full ventilation.
CPAP uses pressure to send a blend of air and oxygen into the mouth at a steady rate, thereby boosting the amount of oxygen that enters the lungs.
But Prof Hill said many Covid-19 patients eventually "progress to invasive ventilation".
This is for people whose illness is so severe they are struggling or unable to breathe for themselves.
A mechanical ventilator either does all the breathing for the patient, or assists the patient's own breathing.
The patient is heavily sedated while a device called an endotracheal tube (ET) is guided through the mouth into the windpipe.
Heavy sedation is then continued because having a tube in the throat can be very uncomfortable.
Patients can be fed at the same time through a tube going into their stomach via their nose.
Prof Hill said it was unclear whether Mr Johnson was breathing on his own, or with the help of a ventilator.
He added: "One of the features of Covid-19 in all countries seems to be that many more men become seriously ill than women - especially in the over 40 age group.
"Also we know that people under about 60 seem to have a higher chance of making a recovery from critical illness with Covid-19 than older people.
"But there is no doubt this turn of events means Boris Johnson is extremely sick.
"It illustrates three of the important healthcare needs of Covid-19.
"Firstly, many patients need help breathing, and there is a shortage of the mechanical ventilators that can do this - and in particular a shortage of the high quality intensive care ventilators most suitable for Covid-19 patients who might need help breathing for over a week.
"Secondly, Covid-19 patients need a huge amount of oxygen to help them breathe - which is potentially going to be in short supply.
"Thirdly, looking after people in intensive care requires skilled staff, and the experience of New York has been that finding enough skilled staff has been the greatest challenge."
The Prime Minister, who celebrated his 55th birthday last June, could also have blood and samples from his throat collected according to guidance issued to doctors by the the World Health Organisation (WHO) last month.
The health body says around 14% of Covid-19 patients will develop symptoms severe enough to need oxygen therapy, and 5% will need intensive care treatment.
The Prime Minister is likely to have already undergone several tests to check his oxygen levels, white blood cell count, and liver and kidney function before he is released from hospital following his admission over Covid-19, according to doctors.
He is also likely to undergo an electrocardiogram to check his heart.
GP Dr Sarah Jarvis told the BBC shortly after he was admitted on Sunday that Mr Johnson would also have his chest X-rayed and lungs scanned, particularly if he was found to be struggling for breath.
The death rate of those admitted to intensive care in the UK with Covid-19 has topped 50%, according to the latest figures.
The figure comes from data compiled by the Intensive Care National Audit and Research Centre (ICNARC) based on a sample of 2,249 coronavirus patients.
The data showed of the 690 patients in the sample whose care outcomes were known, 346 - 50.1% - had died, while 344 had been discharged.
The remaining patients, 1,559, were reported still to be in critical care.
The data shows that of 2,248 patients, 73% were men and 27% were women.
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