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 was speaking as Stormont's First and Deputy First Minister prepare to chair a meeting of the Civil Contingencies Group (NI) today where they will lead the response of the public sector, including emergency services, to the virus.
Parliament Buildings itself has closed to the public.
First Minister Arlene Foster said: "These are unprecedented times and I am well aware of the real concern that exists across our community as the number of confirmed Covid-19 cases increase.
"Further steps are being introduced to keep people safe. We know the burden that some of the necessary measures will have on families and individuals but it is important that they are followed and society pulls together to lessen the impacts of this disease."
Deputy First Minister Michelle O'Neill said: "This is the biggest public health crisis we have faced in a generation. The pandemic continues to escalate globally. Our priority is protecting our people and saving lives, which is paramount."
Yesterday, Mr Johnson 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 who are elderly - will need to undertake even more drastic measures.
The measures were announced as the death toll of people with coronavirus in the UK reached 55.
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.
Last night, Stormont's Executive Office announced a raft of measures would be brought in to mitigate against the worst effects of Covid-19.
Options up for discussion include rates relief for businesses; the continuation of free school meals if schools close; and steps to protect the most vulnerable in society, including the homeless and the elderly.
Ministers have also agreed to special meetings of the Executive to discuss diverting funding into the health service and other departments coping with coronavirus.
On a day of dramatic developments:
Northern Ireland's biggest outdoor sporting event, the North West 200 - due to be run from May 10-16 - is set to be cancelled today.
Belfast City Marathon, scheduled for May 5, has been postponed until September.
Northern Ireland's biggest agricultural show was postponed after coming under intense pressure from livestock exhibitors to cancel this year's event.
A spokesperson from the Royal Ulster Agricultural Society said the Balmoral Show will be postponed until August 19-22.
"We are currently in uncertain times following the recent outbreak of COVID-19 and the health of our stakeholders and visitors remains our key priority," he said.
Mr Johnson said that from today 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.
Across the UK, 1,543 people have tested positive for Covid-19, up from 1,372 at the same time on Sunday. However, many thousands more are estimated to have been infected.