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UK Prime Minister Boris Johnson has urged people to be patient with the lockdown, which is due to last until at least 7 May.
So, what are the rules?
What is a 'reasonable excuse' to go out?
The measures say people should go out as little as possible and only leave home if they have a "reasonable excuse". This includes:
- Exercise - alone, or with members of your household
- Shopping for basic necessities
- Any medical need, or providing care for a vulnerable person
- Travel to or from work, but only when you cannot work from home
What are the rules on exercise?
If you have to go outside stay more than 2m (6ft) apart from anyone other than members of your household. This is called social distancing.
Why is social distancing necessary?
Social distancing is important because coronavirus spreads when an infected person coughs small droplets - packed with the virus - into the air.
These can be breathed in, or can cause an infection if you touch a surface they have landed on, and then touch your face with unwashed hands.
When could the lockdown end?
On 27 April, Mr Johnson said it was still too soon to start easing restrictions as it could risk a "second major outbreak and huge loss of life and the overwhelming of the NHS".
The government has set out five tests for ending the lockdown and it will review the current rules by 7 May.
What is self-isolation?
If you show symptoms of coronavirus - such as a dry cough and high temperature - you must take extra precautions.
You should stay at home and not leave it for any reason.
This is known as self-isolation.
You should not go out even to buy food or medicine, and should order these online, or ask someone to drop them off at your home.
You can use your garden, if you have one.
Who should self-isolate?
Everyone who shows coronavirus symptoms - a fever of above 37.8C, a persistent cough or breathing problems - and everyone who lives in the same home.
- If you live alone, you must stay at home for seven days from the day symptoms start
- If you still have a high temperature after seven days, you must continue to self-isolate until your temperature returns to normal
- However, you do not need to continue to self-isolate if you only have a cough after seven days (a cough can last for several weeks after the infection has gone)
- If you, or someone you live with, develop symptoms, the entire household needs to isolate for 14 days to monitor for signs of Covid-19
- If someone else does become ill during that period, their seven-day isolation starts that day. For example, it might run from day three to day 10 - when that person's isolation would then end. It would not restart if another member of the household fell ill
- Anyone who fell ill on day 13 would have to start a seven-day isolation from that day (spending a total of 20 days at home)
The person with symptoms should stay in a well-ventilated room with a window that can be opened, and keep away from other people in the home.
People are advised not to ring NHS 111 or their GP to report their symptoms unless they are worried.
What about older people and those with health conditions?
The government says people aged 70 and over, and those who have an underlying health condition, should remain at home. They are more likely to be seriously affected by coronavirus.
To minimise the risk, friends or family should drop off food and medicine at the door, or it should be ordered online. GP appointments should be over the phone, or online.
The government says it will work with local authorities, supermarkets and the armed forces to ensure people get supplies of essential food and medicines.
Others in the same household, and carers, can go out as long they observe proper social distancing.