Foreign Secretary Dominic Raab has said he is "confident" Boris Johnson will recover from coronavirus, describing the prime minister as a "fighter".
Speaking at the daily coronavirus briefing, Mr Raab said Mr Johnson remained in "good spirits" and was breathing without assistance.
He also described the PM as not only a colleague - but "also a friend".
Later in the evening, No 10 said the PM remained in critical care but his condition was "stable".
It is understood there will be no further update on Mr Johnson's condition until Wednesday.
Speaking at the daily coronavirus briefing, Mr Raab, who is deputising for the PM, said Mr Johnson was not only a boss but "also a friend".
"All of our thoughts and prayers are with the prime minister at this time, with Carrie, and with his whole family.
"And I'm confident he'll pull through because if there's one thing I know about this prime minister, he's a fighter. And he'll be back at the helm, leading us through this crisis in short order."
It comes as the number of coronavirus hospital deaths in the UK rose to 6,159 - a record increase of 786 in a day, the Department of Health and Social Care said, compared with 439 on Monday.