The prime minister “continues to improve” in intensive care and had a “good night” in hospital, Downing Street has said.
Then, the following day, he was moved to intensive care after his condition worsened.
But the PM looks to be on the mend, with Downing Street saying on Wednesday that Mr Johnson was in “good spirits” and making “steady progress”.
Chancellor Rishi Sunak told that day’s coronavirus news conference that Mr Johnson was now “sitting up in bed” and “engaging positively” with medics.
Former defence minister Tobias Ellwood told Sky News earlier that the PM is “mentally able to make decisions” and remains “accessible” to his team.
Mr Johnson’s spokesman said he is continuing with “standard oxygen treatment”, adding: “The prime minister had a good night and continues to improve in intensive care. He’s in good spirits.”
The PM was last seen in public a week ago, when he joined in with the national round of applause for NHS workers.
Foreign Secretary Dominic Raab, who has been deputising for Mr Johnson, is expected to take part instead.
“The prime minister thanks the NHS for the brilliant care which it is providing,” his spokesman said.
“The claps for carers have provided wonderful, unifying moments for the entire country.
“I’m sure that tonight we will once again see people in their millions paying tribute to our fantastic dedicated care workers.”
Meanwhile, Downing Street has once again emphasised the importance of Britons continuing to follow the coronavirus lockdown over the Easter weekend.
Ministers will carry out a review of the stringent measures next week, with widespread expectation that they will be extended.
“What we absolutely need to do now is keep bearing down on the rate of transmission which will mean continuing with social distancing measures to ensure we are protecting the NHS and saving lives,” the PM’s spokesman said.
“We are at a critical point in this. That is certainly the case as we approach the Easter bank holiday weekend.”
The spokesman praised the “brilliant” public response so far, acknowledging that Britons have “made very big sacrifices in the way they live their lives”.
“You are beginning to see the impact that is having but people really do need to stick with it at what is a critical juncture,” he added.