In a televised address, he said a "moment of national emergency" meant staying at home was necessary to protect the NHS and save lives.

When are people allowed to leave their homes?

Mr Johnson said everybody should stay at home and would only be allowed to leave for specific reasons:

Shopping for "basic necessities", as infrequently as possible. People should use delivery services, where they can
Medical reasons, to provide care, or to help a vulnerable person
Travelling to and from work, but only if it is "absolutely necessary"
People should not meet friends or family members who do not live with them.

What other new measures are there?

The prime minister said:

Shops selling "non-essential goods" including clothes and electronics stores will be closed immediately
Libraries, playgrounds, outdoor gyms and places of worship will also close
Gatherings of more than two people (excluding people who live together) will be banned
There will be no weddings or baptisms but funerals will be allowed
Parks will remain open but people are only to go out to exercise once a day
All of the new measures will last for three weeks from 23 March before being reviewed.

How will the measures be enforced?

The police will be given the powers to make sure people follow the measures - by dispersing groups and issuing fines.

The National Police Chiefs' Council told the BBC that officers would be talking to people and explaining why they shouldn't be out.

If they do not accept the advice, they would then be given a fine. However, fines won't be issued until Parliament passes the emergency legislation - which should be by the end of Thursday.

Downing Street says fines could initially be set at £30, but that could increase significantly if people don't comply.

Councils will also play a role to make sure local businesses follow the rules.

The Local Government Association says that "anyone who sees a business breaching government closure restrictions should report it to their local council so it can investigate and take appropriate action".

What about non-food shops?

Some other types of non-food retailers are also allowed to stay open, according to government guidance:

Pharmacies
Petrol stations
Garages
Bicycle shops, home and hardware shops
Car rentals
Pet shops
Corner shops and newsagents
Post offices
Banks
Laundrettes and dry cleaners
If a customer visits any of these premises, they need to ensure they stick to the social distancing rules.

What about online deliveries?
Customers should still be able to receive online deliveries.

Amazon said there would be some changes, with essential items (like food and healthcare products) being prioritised. That means that delivery times for other items may take longer than usual.

In order to avoid any unnecessary contact, Amazon in-home deliveries have been paused and drivers have been advised to place packages at the customer's doorstep and step back. This means bulky items, like fridges and washing machines, will not be brought directly into homes.

John Lewis says it's still offering home deliveries for larger items such as furniture and home appliances, but will no longer offer home installation or assembly.

The Royal Mail is still operating its service for parcels and letters, but with some delays and alterations.

What else do I need to know?

Parents who live separately will be allowed to move children under 18 between their houses
However, the president of the Family Division of the High Court has clarified that children do not have to move between parental houses, even if contact with the "non-resident parent" is specified by a court order. Andrew Macfarlane said parents can vary the arrangements if they are worried about their child's safety, but would be expected to organise contact by video chat or phone instead

People can still use takeaway shops and delivery companies. However, customers must not eat or drink while waiting
Outside exercise is allowed. However, this can only be done once a day, either on your own, or with the people you live with
The government has announced an automatic six-month extension for all MOTs due for cars, motorcycles and light vans from 30 March onwards. Vehicles must be kept roadworthy, and garages will remain open for repairs
MOTs due before then should be booked, unless you or anyone in your household is self-isolating
MOTs due for lorries, buses and trailers have already been suspended for up to three months
Driving tests in England, Scotland and Wales have been suspended for up to three months, except for critical workers. All practical driving tests are suspended in Northern Ireland until 22 June 2020.

How does the UK compare to other countries?

Many countries have already imposed severe restrictions on their populations.

The whole of Italy has been on lockdown since 9 March, with people only allowed out if strictly necessary
Meetings of more than two have been banned in Germany. One German state, Bavaria, has imposed general restrictions on going outdoors for two weeks
More and more US states have introduced severe restrictions, with one in three Americans now being told not to leave home
Australia has shut down non-essential services, although schools remain open in some states

Why are tougher restrictions being introduced?

The government had already told people to only go out when necessary - for example, to buy food and medicine, or for exercise - and keep 2m (6ft) apart. This is known as social distancing.

However, since this advice was issued, there have been many examples of people not following it.

At the weekend, parks and open spaces were packed with people. Snowdonia National Park, for example, said it had experienced the "busiest ever visitor day in living memory".

Why do people need to stay apart?

Coronavirus spreads when an infected person coughs small droplets into the air. These can be breathed in, or cause an infection if you touch a surface they have landed on, and then touch your face with unwashed hands.