All people in the UK should avoid pubs, clubs and theatres, stop all non-essential contact and travel, and work from home if they can, Boris Johnson has said.
The Prime Minister set out the need for "drastic action" to tackle the "fast growth" of coronavirus across the UK as increased social distancing measures are introduced for the population.
As part of the measures, anyone living in a household with somebody who has either a persistent cough or fever must now also isolate themselves for 14 days.
All people should avoid gatherings and crowded places, while people who are vulnerable - including those are elderly - will need to undertake even more drastic measures.
The Prime Minister said that according to the Scientific Advisory Group for Emergencies (Sage) "it looks as though we are now approaching the fast growth part of the upward curve" in the number of cases.
"Without drastic action cases could double every five or six days," he said.
Mr Johnson said that from Tuesday mass gatherings are something "we are now moving emphatically away from".
He said "if you or anyone in your household" had one of the two symptoms - a high temperature or continuous cough - "you should stay at home for 14 days.
"That means that if possible you should not go out, even to buy food or essentials, other than for exercise and in that case at a safe distance from others."
In a dramatic escalation of the Government's response, Mr Johnson said all members of the public needed to take extra action.
"Now is the time for everyone to stop non-essential contact with others and to stop all unnecessary travel," he said.
Mr Johnson said London appeared to be "a few weeks ahead" of other areas and urged people in the capital to pay special attention to the measures - including home working - suggested by the Government.
