- Video report by ITV News Health Correspondent Emily Morgan
An expert has warned the Prime Minister is "extremely sick" and could require invasive ventilation - a common treatment for coronavirus patients who need intensive care.
Boris Johnson was moved to the critical unit at St Thomas' Hospital on Monday evening as part of his ongoing treatment for Covid-19.
Mr Johnson 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.
Speaking on Tuesday morning, Cabinet minister Michael Gove said the Prime Minister has been given "oxygen support" in intensive care but has not been on a ventilator.
- ICU Dr Alison Pittard outlines the kind of treatment Boris Johnson can expect in hospital
However, if the PM's conditions worsen, Derek Hill, professor of medical imaging at University College London (UCL), said the Prime Minister could be given a breathing aid known as continuous positive airway pressure (CPAP).
The device bridges the gap between an oxygen mask and full ventilation and uses pressure to send a blend of air and oxygen into the mouth at a steady rate, so boosting the amount of oxygen that enters the lungs.
But Prof Hill warned many Covid-19 patients eventually "progress to invasive ventilation" - being put on a ventilator.
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 some form of ventilator.
The professor 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 Organization (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.
- An intensive care consultant explains what ICU is and how patients are treated in critical care:
He is also likely to undergo an electrocardiogram to check his heart.
GP Dr Sarah Jarvis told the BBC shortly after Mr Johnson was admitted on Sunday that the PM would also have his chest X-rayed and lungs scanned, particularly if he was found to be struggling for breath.
Coronavirus: Everything you need to know