Lockdown: What you can (and can't) do
So when can I leave the house?
Effective immediately, the government says you can go outside only for "very limited purposes":
:: Shopping for basics necessities like food and medicine, with Downing Street saying you should do this "as infrequently as possible" and use food delivery services "where you can"
:: One form of exercise a day, such as a run, walk or cycle. This can be done alone or with members of your household.
:: Any medical need, to give care or to help a vulnerable person
:: Travelling to and from work, but only if this is "absolutely necessary" and cannot be done from home
"These four reasons are exceptions," the government's latest guidance says.
"Even when doing these activities, you should be minimising time spent outside of the home and ensuring you are 2m (6.5ft) apart from anyone outside of your household."
If you work in what the government has deemed a "critical sector", or your child has been identified as vulnerable, you can still to take your children to school.
What about seeing friends and family?
If your friends ask to meet you, you should say no.
You should also not be meeting members of your family who do not live in your home.
What else did the PM announce?
All shops selling "non-essential" goods will be closed, as will a range of public spaces and venues.
Stores selling clothing and electronic stores
Hair, beauty and nail salons
Libraries, community centres and youth centres
Playgrounds, outdoor gyms and sports courts
Places of worship
Outdoor and indoor markets, excluding food markets
Bowling alleys, arcades and soft play facilities
Hotels, hostels, bed and breakfasts, campsites, caravan parks, and boarding houses for commercial/leisure use. This excludes permanent residents and key workers.
Public gatherings of more than two people are banned.
There are two exceptions to this, according to the government's guidance:
Where the gathering is of a group of people who live together. For example, this means a parent can take their children to the shops if there is no option to leave them at home
Where it is essential for work purposes. The guidance states that workers should try to minimise all meetings and other gatherings in the workplace.
All social events, including weddings, baptisms and other ceremonies, are also be banned - but funerals can go ahead with immediate members of family in attendance.
Parks will stay open for exercise, but gatherings there will be dispersed.
How will the government enforce this?
Police will have powers to disperse gatherings, while anyone who is found not to be following the rules could be fined.
How long will these restrictions last for?
The PM said the restrictions will be "kept under constant review" and last for a minimum of three weeks.
At that point, the government will examine the evidence and see if they can be relaxed.