Boris Johnson has warned the NHS could be "overwhelmed" if people do not act to slow the "accelerating" spread of coronavirus, as he urged people not to visit loved ones on Mother's Day.
The PM called on people to join a "heroic and collective national effort" and follow social distancing advice.
The number of people who have died in the UK with coronavirus rose to 233 on Saturday, as cases topped 5000.
It comes as NHS England plans to write to 1.5 million people most at risk.
Those at-risk people 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, 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.
"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 added he recognised the government was imposing measures "never seen before either in peace or war" - but said they were essential.
'Cannot sugar-coat threat'
As families prepare to celebrate Mother's Day on Sunday (UK time), Johnson said the best single present for mothers was to stay away.
It comes after the government this week told all restaurants, cafes and pubs - as well as some other public spaces like gyms and cinemas - to close.
"This time, the best thing is to ring her, video call her, Skype her, but to avoid any unnecessary physical contact or proximity," the prime minister said.
"And why? Because if your mother is elderly or vulnerable, then I am afraid all the statistics show that she is much more likely to die from coronavirus, or Covid-19. We cannot disguise or sugar-coat the threat."
On Friday, Johnson was asked at his daily press conference whether he would be visiting his own mother, who is 77. He said he would "certainly be sending her my very best wishes and hope to get to see her".
A Downing Street source later said his contact with his mother on Sunday would be over Skype.
Armed Forces called in to help vulnerable
Meanwhile, the government has said members of the Armed Forces will help ensure essential items like groceries can be delivered to people who are at-risk.
Those identified as the most vulnerable in their communities will be contacted directly as a priority.
Dr Paul Johnstone, from Public Health England, said: "The NHS are contacting the people who are most vulnerable to developing a very serious illness as a result of Covid-19 with specific advice to stay at home for at least 12 weeks.
"If you receive a letter it is vitally important that you act on it for your own protection, don't attend any gatherings of friends or families and don't go out for shopping, leisure or travel."
Elsewhere, Mayor of London Sadiq Khan echoed Johnson's call for social distancing, and described the coronavirus pandemic as the "biggest health, social and economic emergency we have faced since the second world war".
Writing in The Observer, Khan said while the UK was at the beginning of attempts to halt the viruses spread, "we will be living with the consequences for many years to come".
Retired doctors, nurses respond to plea
In the last 48 hours, thousands of retired medics have answered the government's call to return to work to help with the outbreak - including 4000 nurses and 500 doctors.
Health Secretary Matt Hancock paid tribute to the "brilliant support", but said the health service needed "many more" medical workers.
"The whole country needs the NHS right now. And if you're a retired doctor or a retired nurse, then your NHS needs you."
Earlier, the health service announced it had struck a deal with private hospitals to get hold of thousands of extra beds, ventilators and medical staff.
The agreement will see the private sector reallocate almost its entire national hospital capacity to the NHS. Under the terms of the deal, the private sector will be reimbursed at cost, meaning no profit will be made for doing so.