British Prime Minister Boris Johnson warned he would have to take "unprecedented steps" to slow the spread of Covid-19 as the UK death toll from the pandemic rose by 48 in a day to 281.
Around 1.5 million "vulnerable" people will be urged to self-isolate for a period of 12 weeks.
"When we now have to take special steps to protect the particularly vulnerable and you'll remember that I said the moment would come to shield those with serious conditions - there are probably about one and a half million in all.
"This shielding will do more than any other single measure that we are setting out, to save life," Mr Johnson said.
He continued: "The reason we're taking these unprecedented steps ... is, of course, that we have to slow the spread of the disease and to save thousands of lives."
British Communities Minister Robert Jenrick confirmed that the government will tell 1.5 million of its most vulnerable citizens they must stay at home to protect themselves from the coronavirus.
Mr Jenrick said: "I don't underestimate what we're asking of people. It will be tough.
"Today, we have to go further and shield the most clinically vulnerable people to save their lives," he added.
Mr Johnson warned he may impose tougher controls on the British public as packed parks, markets and cafes at the weekend showed thousands of people defying government warnings about social distancing.
"Please follow the advice and don't think that the fresh air gives you some immunity," he said. "You have to stay two meters from each other."
The government would take "additional measures" if people continued to fail to comply, and ministers were keeping options under review, he added.
"If people won't do it, we'll bring in tougher measures."
Around 80% of cases of Covid-19 will be a mild to moderate illness, close to 14% have severe disease and around 6% are critical.
Generally, you need to be 15 minutes or more in the vicinity of an infected person, within 1-2 metres, to be considered at-risk or a close contact.
Earlier, Mr Johnson had warned that Britain's National Health Service (NHS) could be "overwhelmed" by the coronavirus like the Italian health system in just two weeks.
In a statement released by his office, Mr Johnson again urged people to stay at home to stop the spread of the virus.
"The numbers are very stark, and they are accelerating. We are only a matter of weeks - two or three - behind Italy," he said.
"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."
Mr Johnson's government has rapidly stepped up warnings and actions to combat the virus over the past week.
It has closed schools for most pupils, told pubs and restaurants to shut and asked people to avoid unnecessary social interactions - seeking to slow the rise of cases and enable intensive care units in the healthcare system to cope with the epidemic.
Yesterday, the NHS struck a deal with the independent hospital sector, giving it more ventilators and thousands of extra beds and healthcare staff on hand from next week.
Housing minister Robert Jenrick said on Sky News the government had received prototypes of new ventilators – the machines needed to keep the most critically ill alive – and should begin production of the machines soon.
In a separate interview with the BBC he said that Britain needed to go further with testing, adding that it would order millions of an "antibody test" for those who have had the virus that he expected to be ready in the coming weeks.