said today the PM was given oxygen treatment after arriving there by car, but Downing Street refused to comment on his specific treatment.
They said this morning that he had a "comfortable night" in hospital and was "under observation" by doctors.
No10 stressed it was not an emergency trip and was a planned visit on advice from his doctor.
He is still suffering from a cough and a temperature.
A spokesperson said earlier: "The issue is that his symptoms have remained persistent.
"He remains in hospital under observation.
"Any information which the PM needs to be updated on or any decisions that require action from the PM, that is what will happen.
"He is receiving updates in hospital and continues to receive a red box."
The PM has been in touch with No10 colleagues during his stay, and it's not yet known when he will be returning home.
He will keep in touch with his doctors and be guided by them if he is too unwell to work.
Officials didn't comment on whether the PM was in a private ward or sharing with other patients.
The admission was a “precautionary step”, officials said yesterday.
Dominic Raab gave the daily briefing today at No10 - insisting the PM was still in charge Carrie Symonds also developed symptoms of the disease last week Credit: AFP or licensors Dr Rupert Beale, from the Francis Crick Institute, said Mr Johnson is likely to be monitored for signs such as oxygen saturation.
He said: “They will also check blood tests to see what the immune response to the virus looks like, and to assess liver and kidney function.
“They will perform an electrocardiogram to check the heart.
“More sophisticated tests may include a CT scan of the chest to get an accurate picture of the lungs.
“They will consider the best way to deliver oxygen and will also consider other treatments depending on test results."
GP Dr Ellie Cannon said the most common reason coronavirus patients are admitted to hospital was for "signs of breathlessness".
She added that having the virus beyond ten days is a "red flag" and some patients who failed to get rid of the symptoms had developed a bacterial infection on top of the virus.
Dr Cannon told Sky News: "The breathlessness is a worrying sign of severe illness and can happen at any point of the illness, it can happen on Day seven or Day ten."
Ministers urged Boris to oficially step down from leading the charge today and focus on getting better.
James Duddridge MP said: "Take care boss. Get well. Come back fighting. But for now rest, look after yourself and let the others do the heavy lift."
Meanwhile, it was reported this afternoon that Labour frontbencher Tony Lloyd has been hospitalised over coronavirus too.
The Shadow Scottish and Northern Ireland Secretary is in hospital in Manchester but is said to be stable.
Michael Gove's wife Sarah Vine tweeted last night that he was working flat out despite being ill.
She said: "Boris has worked non-stop throughout his illness - and now we see the result."
And a Cabinet minister told the Financial Times: "This may be the jolt, the wake-up call that he realises he has to rest to recover.
"He will have been very reluctant to go to hospital."
Communities Secretary Robert Jenrick said this morning he has heard the PM is "doing well".
He told Radio 4: "He has worked phenomenally hard.
"I know for him personally it must have been very frustrating for him to go into hospital for these tests when he wants to be leading from the front.
"He will updated regularly while he in hospital as he has done when he is self isolating."
Aides said he would stay in for as long as he needs to.
Last night, US President Donald Trump said the whole of America was praying for a speedy recovery for Mr Johnson.
He said at a press conference in the White House: "I want to express our nation's well wishes to Prime Minister Boris Johnson as he wages his own personal fight with the virus.
"All Americans are praying for him, he's a friend of mine, he's a great gentleman and a great leader.
"He was brought to the hospital today but I'm hopeful and sure he will be fine. He's a strong man, strong person."
New Labour leader Sir Keir Starmer and Tory leadership rival Jeremy Hunt were among many British politicians who wished him well.
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His voice sounded croaky and he appeared exhausted.
But she said she was “on the mend” after spending a week in bed.
She tweeted on Saturday night: “I haven’t needed to be tested and, after seven days of rest, I feel stronger and I’m on the mend. Being pregnant with Covid-19 is obviously worrying.”