The PM, who today chaired an emergency COBRA meeting, confirmed the government was now moving to the “delay” stage of the process but would not be closing schools and banning sporting events.
He said “the scientific advice” was that taking such drastic steps “could do more harm than good”.
The official health advice was updated, however, with people with mild symptoms, such as a cough, told to stay at home and take precautions.
As it stands, there are 596 confirmed cases in the UK and ten people have died. Sir Patrick Vallance, the government’s chief scientific adviser, however, said the actual number of people infected could be between 5,000 and 10,000.
Speaking at a press conference in Downing Street, he said: “We’ve all got to be clear, this is the worst public health crisis for a generation.
“Some people compare it to seasonal flu, alas that is not right.
“Due to the lack of immunity this disease is more dangerous.
“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.”
It came as Public Health England issued new stay-at-home guidance for those with confirmed or suspected cases of coronavirus.
It states not to go to work, school, or public areas, use public transport or taxis – or even go for a walk.
Even those with mild symptoms of infection are being urged to stay at least two metres away from other people, to stay well away from elderly people and to sleep alone.
Where possible, those with confirmed or suspected cases should use a separate bathroom from the rest of the household, PHE said.
For infected people with shared toilets and bathrooms, the government advice says to clean them after every use.
Johnson went on to warn that the peak of the outbreak had yet to be reached and that the most dangerous period is some weeks away.
He stressed that the “lines of defence” must be deployed at the right time to maximise their effect.
He said: “The most important task will be to protect our elderly and most vulnerable people during the peak weeks when there is the maximum risk of exposure to the disease and when the NHS will be under the most pressure.
“So the most dangerous period is not now but some weeks away depending on how fast it spreads.”
The PM added: “There is no escaping the reality that these measures will cause severe disruption across our country for many months.
“The best scientific advice is that this will help us slow the disease and save lives.”
He urged people to use the 111 service online rather than call the helpline.
In a message to the elderly, Johnson said that although the disease was “particularly dangerous” for them, the vast majority would experience “a mild-to-moderate illness”.
Calling for action to support older relatives and neighbours, Johnson said: “We are going to need to mobilise millions of people to help and support each other.”