In stark message to Britain on Saturday, the prime minister said the National Health Service was in danger of being "overwhelmed" in the same way as the Italian healthcare system unless people heeded government advice on "social distancing".

It came as ministers urged the 1.5 million people in England considered to be most at risk from the disease due to their health conditions to begin "shielding" themselves by staying at home.

Letters will go out this week to those concerned, "strongly advising" them not to go out for at least 12 weeks from Monday.

At the same time, the government 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.

It follows the dramatic announcement on Friday that all pubs, restaurants, cinemas and theatres are to shut in the latest move to combat the disease.

Johnson acknowledged they were imposing measures "never seen before either in peace or war", but said they were essential as the outbreak gathered pace.

"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," he said.

"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."

Johnson said that while "everyone's strongest instinct" was to visit their mother on Mother's Day, the best single present they could give her was to stay away and spare her the risk of becoming infected.

"This time the best thing is to ring her, video call her, Skype her, but to avoid any unnecessary physical contact or proximity," he 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."

The latest official figures released on Saturday showed the number of people across the UK who have died after testing positive for Covid-19 has risen to 233, with 53 more deaths in England, two in Wales and one in Scotland.

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 (COPD), and with some cancers such as those of the blood or bone marrow.

They also include some - though not all - of those receiving certain types of drug treatments including those which suppress the immune system - leaving the body less able to fight off the virus.

Australian Associated Press