Transmission of Covid-19 is happening more rapidly in London, the Prime Minister has said.
Addressing the UK, Boris Johnson said London is weeks ahead of other regions in terms of the virus curve, meaning transmission is happening more rapidly.
He told Londoners to pay special attention to the advice to work from home and to avoid unnecessary social contact.
Pubs, clubs and theatres should no longer be visited, he added.
As of 16 March, London has 480 confirmed coronavirus cases. A total of 55 people have died in the UK due to Covid-19 - 14 of those were from London.
"The very draconian measures outlined will be asking a lot from the everyone", the PM said.
"What we're doing is giving very strong advice that public venues such as theatres should no longer be visited.
"It's important that Londoners now pay special attention to what we're saying about non-essential contact and to take particularly seriously the advice about working from home and avoiding confined spaces such as pubs and restaurants."
His comments come after news passenger numbers on the London Underground have declined 19% during the outbreak.
A shutdown of the West End and other theatres around the country is also likely.
The Mayor of London Sadiq Khan cancelled the upcoming St Patrick's Day celebrations and Buckingham Palace announced The Queen cancelled a planned visit to Camden on 26 March.
The mayor also told BBC London the Tube would run a Saturday service on weekdays.
"You should avoid pubs, clubs, theatres and other such social venues," Mr Johnson said in his first daily news briefing on Monday.
"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."
Mr Johnson added that by the weekend those with the most serious health conditions will be shielded from social contact for 12 weeks.
Mr Khan said he supported the advice to Londoners and hopes the measures will reduce the chance of transmission.