Johnson was admitted to St Thomas'' Hospital on Sunday, on the advice of his doctor, after continuing to have a cough and high temperature 10 days after testing positive for the virus.
The Prime Minister was given oxygen before being taken to intensive care on Monday. He was currently "responding to treatment" and remained clinically stable.
The government has reportedly been told that the country's deadliest day could be April 18, meaning that Britons will be told to stay inside until at least next month, according to the newspaper.
The news comes after the UK death toll jumped by 938 people in just one day, taking the total number of fatalities to 7.097, while the number of confirmed coronavirus cases increased to 61,474.