The UK is on Friday to introduce more stringent measures to fight coronavirus as it moves from the “contain” phase to “delay”.
People with coronavirus symptoms such as a persistent cold or high temperature “however mild” have been told not to leave their house for seven days.
So far, 590 people in the UK have tested positive for coronavirus, with 10 deaths.
He warned: "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."
Mr Johnson described the coronavirus outbreak as the “worst public health crisis for a generation” that would cause clear disruption in the coming months.
Coronavirus: What is a pandemic?
Addressing the nation, Prime Minister Boris Johnson said his government advised against international school trips and was considering banning major public evens such as sporting fixtures. Schools, however, will not be closed at this time.
"British nationals aged 70 and over, and those with underlying health conditions such as chronic diseases and diabetes, have been advised not to travel on cruise ships in response to the coronavirus outbreak," Britain's Foreign Office said.
The Prime Minister was speaking after chairing a meeting of senior UK officials.
The UK prime minister said it was more vital than ever for individuals to wash their hands. He added the UK would "get through this” a so long as the country stood together.
"There are currently about 590 cases identified in the UK and there are more than 20 patients on intensive care units," chief scientific adviser Patrick Vallance said.
"If you calculate what that really means in terms of the total number, it is much more likely that you have somewhere between 5,000 and 10,000 people infected at the moment."
In its official advice to those self-isolating, the government says:
"You should remain in your home. Do not go to work, school, or public areas, and do not use public transport or taxis. You cannot go for a walk.
"You will need to ask friends or relatives if you require help with buying groceries, other shopping or picking up medication. Alternatively, you can order medication by phone or online.
"You can also order your shopping online. Make sure you tell delivery drivers to leave items outside for collection if you order online. The delivery driver should not come into your home
Earlier, Scotland’s First Minister Nicola Sturgeon laid out her devolved government's stance on the coronavirus emergency.
“The decision has been taken that we have now moved from a contain phase into the delay phase,” she said.
“The objective is to seek to slow down the spread of the virus, to reduce the numbers who will be infected at the peak," the first minister added.
Face-to-face talks between UK and European Union officials scheduled for next week will not happen in London.
