The PM is being kept at St Thomas' Hospital in London "for close monitoring", Downing Street said.
Mr Johnson's condition is "stable" and he remains in "good spirits", his spokesman added on Tuesday evening.
Downing Street also confirmed that the planned review into whether the UK's coronavirus restrictions could be eased would not go ahead on Monday next week.
Foreign Secretary Dominic Raab, who is deputising for the PM, has said he was "confident" the PM would recover from this illness, describing him as a "fighter".
Speaking at Tuesday's Downing Street coronavirus briefing, he said Mr Johnson was receiving standard oxygen treatment and was breathing without any assistance, such as mechanical ventilation or non-invasive respiratory support.