The prime minister suggested families “avoid any unnecessary physical contact” and try to protect their loved ones by speaking over the phone or online instead.
It comes as Mr Johnson warns the nation is ”only a matter of weeks – two or three – behind Italy”, where officials reported 793 deaths across a 24 hour period on Saturday to bring the total to more than 4,800.
“If your mother is elderly or vulnerable, then I am afraid all the statistics show that she is much more likely to die,” he said. “We cannot disguise or sugar-coat the threat.”
Having already ordered the closure of all pubs, restaurants, cinemas and theatres, Mr Johnson said the growing number of restrictions on daily life were essential to slow down the spread of the virus.
“The numbers are very stark, and they are accelerating,” he said. ”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.”
The latest in a string of initiatives from the government has seen letters sent out to the 1.5 million people in England considered to be most at risk, advising them not to leave their homes for at least 12 weeks from Monday.
Those being urged to stay at home include people who have received organ transplants, those severe with respiratory conditions such as cystic fibrosis and severe chronic bronchitis, and with some cancers such as those of the blood or bone marrow.
Where possible, they will receive regular text messages containing advice and guidance on how to manage their condition while at home, including on getting prescriptions delivered and accessing support for daily living.
Anyone living with them is also urged to “stringently” minimise any personal contact.
Dr Paul Johnstone, director of Public Health England, urged people contacted by the NHS not to ignore the advice. ”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,” he said.
Ministers have also announced a new local support system to ensure people self-isolating at home without the support of family or friends can get basic groceries delivered.
“This will be an especially worrying time for those with serious underlying health conditions and that is why we are urgently acting to ensure extremely vulnerable individuals are taking extra steps to shield themselves, and that the essential items they need are supplied to them,” said communities secretary Robert Jenrick
“We will ensure that vulnerable and older people in our society are left in no doubt of their importance to us and our determination to protect them as best we can.”
Additional reporting by Press Association
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