Letters are being sent to 1.5 million people most at risk of coronavirus, urging them to stay at home.
It comes as the PM asked the UK not to visit loved ones on Mother’s Day, warning the NHS could be “overwhelmed”.
Boris Johnson has called on the public to join a “collective national effort” and follow social distancing guidance.
Mayor of London Sadiq Khan appealed for people to heed the advice, saying “do it for loved ones who will die if you don’t”.
The number of people who have died in the UK with coronavirus rose to 233 on Saturday, as cases topped 5,000.
People who are most at risk will receive letters or text messages strongly advising them not to go out for 12 weeks to protect themselves, the government said.
They include people who have received organ transplants, are living with severe respiratory conditions such as cystic fibrosis or specific cancers, such as blood or bone marrow.
In a message to the country on Saturday evening, Mr Johnson said: “The numbers are very stark, and they are accelerating.
“The Italians have a superb health care system. And yet their doctors and nurses have been completely overwhelmed by the demand.
Chart showing the number of coronavirus deaths by region
“The Italian death toll is already in the thousands and climbing. Unless we act together, unless we make the heroic and collective national effort to slow the spread – then it is all too likely that our own NHS will be similarly overwhelmed.”
He said the UK is only “two or three” weeks behind Italy, adding that he recognised the government was imposing measures “never seen before either in peace or war” – but said they were essential.
There have been 220,000 cases of the virus worldwide with more than 9,000 deaths.
Italy has seen its death toll for the past month reach 4,825, the highest in the world.