Prime Minister Boris Johnson said anyone who lives with someone who has a cough or a temperature should stay at home for 14 days.
He added “now is the time for everyone to stop non-essential contact and to stop all non-essential travel” as he urged people to work from home and avoid pubs, clubs and theatres.
Boris Johnson set out the need for “drastic action” to tackle the “fast growth” of coronavirus.
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 added “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,” he said.
In a dramatic escalation of the Government’s response, Mr Johnson said that even those without symptoms 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 that from Tuesday the Government would no longer be supporting mass gatherings with emergency workers.
Mass gatherings are something “we are now moving emphatically away from”, he said.
Mr Johnson added: “We want to ensure that this period of shielding, this period of maximum protection, coincides with the peak of the disease and it is now clear that the peak of the epidemic is coming faster in some parts of the country than in others.
“And it looks as though London is now a few weeks ahead.”
Mr Johnson said that it is “important that Londoners now pay special attention about avoiding non-essential contact and to take particularly seriously the advice about working from home and avoiding confined spaces such as pubs and restaurants”.
He added: “Lastly, it remains true – as we said in the last few weeks – that this sort of transmissions of the disease at mass gatherings such as sporting events are relatively low, but obviously, logically, as we advise against unnecessary social contact of all kinds, it’s right that we should extend that advice to mass gatherings as well.
“And so we’ve also got to ensure that we have the critical workers we need that might otherwise be deployed for those gatherings, to deal with those emergencies.
“So from tomorrow we will no longer be supporting mass gatherings with emergency workers in the way that we normally do.”