Foreign Secretary Dominic Raab has said that Boris Johnson is a "fighter" who "will pull through" after spending the night in intensive care.
The Prime Minister was moved to intensive care on Monday night following a worsening of his coronavirus symptoms.
Speaking at the daily Covid-19 press conference, Mr Raab, who is deputising for the PM, said that Mr Johnson remains in "good spirits" after spending another night at St Thomas' hospital.
The Foreign Secretary said: "He is not just the Prime Minister. For all of us in Cabinet, he is not just our boss.
"He is also a colleague and he is also our friend. So all our thoughts and prayers are with the Prime Minister at this time, with Carrie (Symonds) and his whole family.
"And I'm confident he will pull through because if there is one thing that I know about this Prime Minister is he is a fighter and he will be back leading us through this crisis in short order."
Mr Raab, the Foreign Secretary and First Secretary of State, told the daily Downing Street coronavirus press conference: "He's receiving the very best care from the excellent medical team at St Thomas's Hospital.
"He remained stable overnight.
"He's receiving standard oxygen treatment and breathing without any assistance, he's not required any mechanical ventilation or non-invasive respiratory support.
"He remains in good spirits and in keeping with usual clinical practice his progress continues to be monitored closely in critical care."
Mr Raab is standing in for the Prime Minister "whenever necessary", including leading the daily meetings of the coronavirus "war cabinet".
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He said he had "total confidence" in the arrangements the Prime Minister had put in place to allow the Foreign Secretary to deputise for him.
The Cabinet has "very clear instructions" from the Prime Minister whilst he remains in hospital, Mr Raab added.
"Well first of all, decision making by Government is made by collective Cabinet responsibilities, so that is the same as before.
"But we've got very clear directions, very clear instructions from the Prime Minister, and we're focused with total unity and total resolve on implementing them so that when he's back, I hope in very short order, we will have made the progress that he would expect and that the country would expect."
