A Northern Ireland consultant in infectious diseases has said Prime Minister Boris Johnson could be ventilated for up to two weeks in an intensive care unit if he needs the treatment after his coronavirus symptoms worsened.
Cabinet Office Minister Michael Gove said Mr Johnson, who is 55, is not on a ventilator but has had some oxygen support and if his condition changes the government will make an official statement.
In a statement on Tuesday, a No 10 spokesperson said the Prime Minister was stable overnight and "remains in good spirits".
"He is receiving standard oxygen treatment and breathing without any other assistance," they said. "He has not required mechanical ventilation or non-invasive respiratory support."
Northern Ireland Secretary of State Brandon Lewis has said the PM is "fully conscious".
Dr Cillian Quinn, who himself overcame Covid-19, said that the recovery rate for those who fall ill from the virus is "very variable" and depends on their state when they were first taken to intensive care.
"It's probably too early in the course of the epidemic to make definitive measurements in terms of who recovers," he said on BBC's Good Morning Ulster.
"We do know that when people are ventilated, that they tend to be ventilated for quite a long time and that can be for up to 14 days in some circumstances."
He said that while Boris Johnson may have been reasonably well shortly after arriving in St Thomas' Hospital in central London, staff often see conditions of patients rapidly deteriorate.
The Prime Minister spent a seven-day period of isolation in Downing Street before being admitted to hospital.
Dr Quinn said: "There's generally a fairly rapid increase in oxygen requirements over a number of hours, something like 12 hours or so and that generally manifests with patients looking reasonably well and sitting comfortably on wards but what you tend to notice is the oxygen requirements are gradually going up.
"We know once we get to certain oxygen requirement of around 60%, we would definitely be getting involved with intensive care colleagues to provide extra support to patients.
"Some patients can deteriorate very, very rapidly with a hyper-inflammation type syndrome and that can develop further with increasing respiratory infiltrates and often what looks like on a chest x-ray something similar to heart failure," said Dr Quinn, who works in London but is originally from Silverbridge in Co Armagh.
The doctor, who suffered from mild symptoms including chest tightness and a cough, said Mr Johnson is currently sitting in "the danger category", which usually takes place around seven to 10 days after the onset of symptoms.
"No patient is taken to intensive care lightly so obviously his oxygen requirements have increased quite significantly and he is going to need some extra support.
"Whether that will get to the point where he needs ventilation or not, who knows - but it had clearly got to a situation where he couldn't be managed on a ward level."
The news that Taoiseach Leo Varadkar, a qualified doctor, is to spend one day a week working again in the health service as Ireland does battle with the coronavirus has been met with cynicism in some quarters.