The Department of Health and Social Care said there were now 51,608 confirmed coronavirus cases.
Johnson said this morning he was in "good spirits".
Prime Minister Narendra Modi on Monday wished his British counterpart an early recovery after the latter was on Sunday hospitalised with the highly infectious Covid-19.
"We were told [when Johnson was admitted on Sunday] that he would receive routine checks - they could involve checking his lungs, but beyond that, there's been very little detail of what treatment he has received and what condition he has been in".
President Trump, meanwhile, said "all Americans are praying" for the prime minister.
The 55-year-old leader had been quarantined in his Downing Street home since he was found to have COVID-19 on March 26.
It's an extraordinary turn of events for Johnson.
"We will be with our friends again, we will be with our families again, we will meet again", she said.
"That's the way we'll bring the whole country through the coronavirus challenge", he said.
After he entered the hospital on Sunday, Downing Street faced questions as to why Johnson had not handed authority to Raab - the second highest-ranking government official - on an interim basis.
"The reasons some people get seriously ill with COVID-19 while others have minor symptoms is not yet fully understood, ?"
Johnson himself was criticized by medical experts and members of his own Conservative Party for failing to act quickly enough to close schools and ban public gatherings.
Raab later conceded he hadn't spoken to Johnson since Saturday.
Johnson has faced criticism for initially approving a much more modest response to the coronavirus outbreak than other European leaders, saying on March 3 he had shaken hands with coronavirus patients.
In the LBC interview, Gove said Raab would be chairing Cabinet meetings on Tuesday. He plays tennis and while mayor of London used to cycle around the capital.
Trade secretary Liz Truss said she was wishing for a speedy recovery.
At a press conference less than two hours before the decision was made, Mr Raab told a press briefing that the PM remained in charge of the Government despite remaining under observation. At times, though, he appeared short of breath and visibly unwell. "I'm in good spirits and keeping in touch with my team, as we work together to fight this virus and keep everyone safe".
Yet Johnson's team gave few details when asked what had changed.
New Labour leader Sir Keir Starmer described it as "terribly sad news". Worldwide leaders including Irish premier Leo Varadkar and President Emmanuel Macron of France also sent messages of support. And Johnson's pregnant fiancée, Carrie Symonds, said she has been suffering with symptoms of COVID-19 for the past week.