Johnson announced that the UK is approaching the “fast growth part of the upward curve” of the outbreak of the disease, and warned that “drastic action” is necessary to prevent the number of cases from potentially doubling every five or six days.
He said: "Now is the time for everyone to stop non-essential contact with others and to stop all unnecessary travel.”
Government advice now instructs anybody who lives with somebody who has a cough or temperature to stay at home for 14 days.
It also says that people should work from home if at all possible.
Social contact in public places is also advised against, with pubs, clubs, restaurants and venues best avoided.
The overstretched NHS is also a concern, and people are advised to only use NHS services when they absolutely need to.
The government also said it would not be “supporting” mass gatheirngs with emergency service workers from tomorrow (Match 17).
UK chief scientific adviser Sir Patrick Vallance said the social distancing measures laid out "are not easy", but says "they will have the effect if we all do it."
He added that it "may be necessary" to think about school closures.
There have been a total of 1,543 positive tests for coronavirus in the UK as of 9am on Monday, up from 1,372 at the same time on Sunday.