Boris Johnson has been given "oxygen support" in intensive care but has not been on a ventilator, a Cabinet minister has said.
The Prime Minister was taken to St Thomas' Hospital in London on Sunday night "as a precaution" after his coronavirus symptoms persisted.
However, Mr Johnson's condition worsened on Monday afternoon and he was taken to an intensive care unit (ICU).
Speaking on Tuesday morning, Michael Gove gave an update on the PM's condition, telling LBC: “He is not on a ventilator.
"The Prime Minister has received some oxygen support.
“He is kept, of course, under close supervision. By being in intensive care if there is further support he needs it is there at hand. But the Prime Minister has not been on a ventilator.”
Oxygen support is given to patients who are having difficulty breathing.
Speaking on BBC Radio 4, Mr Gove said he was "not aware" of whether the diagnosis from medics was that Mr Johnson has pneumonia.
ITV News Political Editor Robert Peston reported on the situation shortly after Mr Johnson was admitted to intensive care
Downing Street said Foreign Secretary and First Secretary of State Dominic Raab would "deputise where necessary" while the Prime Minister was in intensive care.
On Tuesday and in the absence of the PM, Mr Raab chaired the Government's daily emergency coronavirus meeting.
Dominic Raab arrives at Number 10 to chair the coronavirus meeting
A spokesperson for No.10 said on Monday evening: “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.”
The Queen has been kept informed about the Prime Minister's condition by Downing Street, Buckingham Palace said.
Mr Johnson was moved to the ICU (intensive care unit) at St Thomas’ at around 7pm on Monday night after his condition worsened over the course of this afternoon, the spokesperson said.
He has been moved to the ICU as a precaution should he require ventilation to aid his recovery, they said.
The Prime Minister "remains conscious at this time", the spokesperson said.
Speaking on Monday evening, Mr Raab said the Prime Minister was in "safe hands" and was receiving "excellent care" at St Thomas' Hospital.
"The Government's business will continue," he said.
"The focus of the Government will continue to be on making sure the prime minister's plans for making sure we can defeat coronavirus and pull the country through this challenge will be taken forward," the Foreign Secretary said.
Mr Raab added there was an "incredibly strong team spirit behind" Boris Johnson and that ministers were determined to carry out his "direction".
"And that's the way it will bring the whole country through the coronavirus challenge that we face right now," he added.
ITV News Political Editor Robert Peston on what this means for the Government in the middle of the coronavirus crisis?
Politicians from all-parties expressed their support for the Prime Minister.
Mr Johnson's predecessor Theresa May tweeted: "My thoughts and prayers are with @BorisJohnson and his family as he continues to receive treatment in hospital. This horrific virus does not discriminate. Anyone can get it. Anyone can spread it. Please #StayHomeSaveLives."
Newly elected Labour leader Keir Starmer tweeted: "Terribly sad news. All the country’s thoughts are with the Prime Minister and his family during this incredibly difficult time."
Scotland First Minister Nicola Sturgeon tweeted: "My thoughts are with the PM and his family - sending him every good wish."
Chancellor Rishi Sunak tweeted: "My thoughts tonight are with @BorisJohnson and @carriesymonds. I know he'll be getting the best care possible and will come out of this even stronger."
Professor Linda Bauld, Bruce and John Usher Chair in Public Health at the Usher Institute, University of Edinburgh, said the admission of the Prime Minister to intensive care shows how "indiscriminate" the virus is.
She said: "Anyone anywhere, including the most privileged in our society, can be affected and can become seriously ill. It is imperative now, more than ever that the rest of us comply with government guidelines to stay at home and not put others at risk.
"Questions will be asked in future about whether the UK government acted appropriately in keeping Parliament open and face to face meetings going while the rest of the country was already following advice to shut down.
"For now, however, all our thoughts will be with the Prime Minister and his family, and the many other families who are facing similar circumstances with critically ill relatives."
Coronavirus: Everything you need to know