Flanked by the CMO and the government chief scientific officer at a Downing Street press conference on Thursday, Mr Johnson confirmed the UK response to the COVID-19 outbreak was now moving on from containment to the 'delay' stage.
A day after the WHO declared coronavirus a global pandemic, the UK prime minister said that from 13 March people with even mild symptoms that include a 'continuous cough or fever' should stay at home in self-isolation for seven days.
People who compare coronavirus to seasonal flu are 'not right', Mr Johnson stressed. 'Owing to lack of immunity this disease is more dangerous and will spread further,' he said - adding that 'more families will lose loved ones before their time'.
Mr Johnson's comments came as official figures confirmed that the total number of confirmed UK cases had risen to 590, including 10 deaths.
The government said has also imposed a ban on international school trips, but has decided against closing schools at this stage. Mr Johnson said a ban on major sporting events was also being considered.
In the next few weeks measures were likely to go further, including asking people with even mild cough or fever to self isolate along with anyone else in their household.
CMO Professor Chris Whitty said all four UK CMOs had agreed to move the UK risk level to 'high' - but warned that the NHS needed to guard against taking more extreme measures too early in efforts to control the spread of COVID-19.
Cough or fever
He said the advice for people with a continuous cough or fever to stay at home was important to help reduce the chance of people with symptoms having contact with older and more vulnerable people.
He added that the measure could help to reduce the peak of the epidemic and push it back further into spring when pressure on the health service from other conditions could ease.
Professor Whitty said that after seven days the vast majority of cases were not infectious - but warned that in the earlier days people with even mild symptoms could spread the illness to 'quite a lot of people'.
The NHS is now shifting its testing strategy for coronavirus cases too, the CMO announced and will no longer attempt to identify every case.
Although the 'geographical limits to who gets tested' will be lifted - so that patients can be tested even if they have not recently visited an at risk country - testing capacity will 'pivot' to focus on patients admitted to hospital.
A further package of measures could be announced at a later stage around social distancing for older people, but it remained too early to press ahead with this now, Professor Whitty said.