They include people with severe respiratory conditions and those suffering from some cancers such as those of the blood or marrow.
With the U.K.'s weekend newspapers full of reports about people meeting in parks and traveling to coastal towns, the prime minister said his government may be forced to take more stringent methods to enforce social distancing, and will consider options over the next 24 hours.
"And so today we have to go further to shield the most clinically vulnerable people to help save their lives".
He said there would be opportunities for members of the public to volunteer.
Adding, however, that it was necessary to protect people who are vulnerable, he said that it was important to note that people living with those identified as vulnerable would not be required to stay at home.
Dr Jenny Harries, deputy chief medical officer for England, was also speaking alongside the prime minister and says it has been a "hugely complex task" to determine and contact the 1.5 million people being asked to stay at home for now.
The UK government will urge the country's most vulnerable citizens to isolate themselves for at least the next 12 weeks in connection with the coronavirus pandemic, Secretary of State for Housing, Communities and Local Government Robert Jenrick said at a press conference on Sunday.
He also said the government had already received some prototype ventilators from the private sector, and has boosted the total number to 12,000 from 5,000 at the start of last week.
There will be a phone number to ring if those isolating need help getting food or medicine.
Free food deliveries will be organised for those stuck inside, as well as medications being brought to their homes.
He says that - despite this - people must follow social distancing advice otherwise "there is no doubt" that the government will bring forward further measures.
The Government and health officials have urged the 1.5 million people in England considered most at risk from the disease due to their health conditions to begin "shielding" themselves by staying at home.
While he said he didn't "want" to enforce "tougher measures", Johnson later reiterated: "You've got to follow the social distancing rule".
People who can't get out to get food and other essential supplies will have parcels delivered to their doorsteps, as the government works with the military on a regional distribution network that will take over pubs and other community hubs.
'Supermarkets are best placed to take action and put in place specific systems and schemes in their own stores, knowing their customers and what's going on, on the ground and many are doing that, helping the elderly have specific times of day to go shopping or have special schemes in place for NHS and social care workers, and I strongly support that.
"But if you are one of these people I want to assure these people on behalf of the Government that you are not alone".