He had been in St Thomas’ Hospital in central London for tests and observation, but his doctors advised he be admitted to intensive care after he experienced breathing difficulties, YahooNews reported.
Johnson was understood to be conscious when he was moved to intensive care at about 7pm, as a precaution should he require ventilation to aid his recovery.
Downing St did not offer an additional update on his condition on Tuesday morning, but media are expecting to be briefed on the prime minister at midday.
His absence leaves a vacuum in leadership at a crucial time in the UK’s response to the COVID-19 pandemic, which so far has killed 5,373 patients.
Dominic Raab will deputise for Johnson ‘where necessary’, on Johnson’s instruction.
The foreign secretary could be set to lead Britain in the battle against coronavirus for some time, with Johnson unable to resume work for an extended period.
Johnson, 55, had been admitted to hospital on Sunday after his coronavirus symptoms had persisted for 10 days for what Downing Street said were precautionary tests.