Video report by ITV News Political Editor Robert Peston
Prime Minister Boris Johnson has said "drastic action" now needs to be taken as he outlined new measures to try and combat coronavirus.
In the first of the Government's daily briefings, Mr Johnson said that "now is the time for everyone to stop non-essential contact and to stop all non-essential travel".
It comes as the number of coronavirus-related deaths in the UK rose to 53.
A total of 1,543 have tested positive for Covid-19 in the UK as of 9am Monday morning.
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.
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Mr Johnson urged the public to work from home when possible and to avoid pubs, clubs and theatre and said that anyone who lives with someone who has a cough or a temperature should stay at home for 14 days and not even venture out for supplies if possible.
According to the Scientific Advisory Group for Emergencies (Sage), he said, "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," the PM warned.
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As a result, from Tuesday, Mr Johnson said 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.
Following the announcement, the Society of London Theatres (SOLT) and UK Theatre venues announced they were closing their member venues. The two groups says they cover "most of the venues in London".
In a statement, the group said West End theatres will "remain closed until further notice".
Mr Johnson said by the weekend, groups particularly vulnerable to Covid-19 will be asked to stay at home for 12 weeks.
He said: "In a few days time, by this coming weekend it will be necessary to go further and to ensure that those with the most serious health conditions are largely shielded from social contact for around 12-weeks.
"Again, the reason for doing this in the next few days rather than earlier or later is that this is going to be very disruptive for people who have such conditions."
"Unnecessary" visits to friends and relatives in care homes should cease, Mr Johnson added.
The prime minister said: "You can take it from what we have just said about avoiding all unnecessary contact for those particular groups - the really strong advice that we are giving to people to avoid unnecessary contact with the over-70s, those with particular health conditions - absolutely, we don't want to see people unnecessarily visiting care homes."
Asked whether pubs, restaurants and theatres were being ordered to close, the prime minister said: "What we are doing is giving very strong advice that public venues such as theatres should no longer be visited.
"The proprietors of those venues are taking the logical steps that you would imagine, you are seeing the change happen already.
"As for enforcement, we have the powers if necessary but I don't believe it will be necessary to use those powers."
Chief medical officer for England professor Chris Whitty, who stood alongside Mr Johnson in the briefing, said measures to tackle the spread of the disease would need to be in place for a “prolonged period”.
He said experts had examined the proposals - looking at mathematical models and behavioural sciences - to find the measures which were the most effective but were also sustainable.
"This is going to go on for some time," he said. "We should not be under any illusions that 'if we just do this for a couple of weeks that is sufficient'.
"This is going to have to be a prolonged period."
The Government's chief scientific adviser Sir Patrick Vallance said other measures may be necessary - including school closures - at some point.
"Those things need to be done at the right time,"
It came as the head of the World Health Organization (WHO) said all countries must "test, test, test" people for Covid-19.
Dr Tedros Adhanom Ghebreyesus, director general of the WHO, urged widespread testing to keep track of the coronavirus pandemic, saying WHO has shipped almost 1.5 millions tests to 120 countries.
The call is in contrast to what is happening in the UK, where only the most serious cases are now being followed up.
In the delay phase of trying to tackle coronavirus in the UK, Public Health England (PHE) and the Government have now moved to a more "targeted approach" to testing.
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