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.
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.