Stressing the need to protect others, Mr Johnson said: 'If you have coronavirus symptoms however mild, either a new continuous cough or a high temperature, then you should stay at home for at least seven days to protect others and help slow the spread of the disease'.
He announced new advice for people aged over 70 - that they should not go on cruises - and said schools should not take pupils on trips overseas.
The UK is expecting the peak in 10 to 14 weeks.
According to PoliticsHome, Nicola Sturgeon said schools won't be closed under new measures - but they would be asked to cancel any trips overseas.
'It is going to spread further and I must level with you, I must level with the British public: many more families are going to lose loved ones before their time'.
However, speaking after Thursday's emergency Cobra meeting, the government's chief scientific officer, Sir Patrick Vallance, said the true total was likely to be between 5,000 and 10,000.
At the same press conference, chief medical officer for England Professor Chris Whitty said the NHS would now "pivot all the testing capacity to identifying people in hospital" rather than mild cases in the community. Health workers would no longer test people for the virus in their homes, but would continue to test people already in hospitals, where the risk of contagion is higher and the impact more serious.
The move is created to delay the peak of the virus to ease pressure on the state-run National Health Service, which is already stretched by winter flu outbreaks.
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However, with Italy in complete lockdown, the United States implementing travel bans and Ireland closing all schools, colleges and gatherings over 100 people, Mr Johnson has faced criticism for not implementing similar measures, with the Cheltenham races entering its third day and no word on a decision to postpone London's St Patrick's Day parade due to take place this Sunday.
Britain's nuanced response to the outbreak contrasted sharply with drastic measures announced by other countries yesterday, which included school closures, flight bans and sports events being abandoned, but Mr Johnson insisted his was the right approach.
He said: "We are not, repeat not, closing schools now".
Pressure mounted for action after neighbouring Ireland, which has had 43 cases and one death, announced that schools, galleries and museums would shut from Friday until March 29.
The British government issued new health advice to the elderly and children on Thursday as the country's coronavirus plan transitions from "contain" to "delay" and the social and economic impact increases.
You do not need to contact NHS 111 to tell them you're staying at home.
"The scientific advice though is this has little effect on the spread - but it does place a burden on other public services".