Face coverings are compulsory from today, 24 July!
They must also be worn when buying takeaway food and drink, although they can be removed in a seating area.
Those who break the rules could face a fine of up to £100.
And police will be able to "use force" to remove customers from shops if they do not wear face coverings, as well as prevent them from entering, guidance from the College of Policing says.
However, forces have said they will only be enforcing the rules, including issuing the £100 fines, as a last resort - and officers will not be patrolling premises.
There are exemptions to the new rules for children under 11, those with disabilities or certain health conditions, such as respiratory or cognitive impairments that make it difficult for them to wear a face covering.
Public Health England has warned parents not to buy coverings for babies and young children because of the risk of choking or suffocation.
It is not compulsory for shop workers to wear face coverings but the government said it "strongly" recommended that employers consider their use where appropriate.
The government said it was the responsibility of individuals to wear one, although businesses are encouraged to take steps to encourage customers to follow the law, such as with signs.
Sainsbury's and Costa Coffee have already said their staff will not challenge or enforce customers who enter their stores. Asda said enforcement was the "responsibility of the relevant authorities".
But Waitrose said it will have staff at the entrance reminding customers of the rule, while Greggs and McDonalds said takeaway customers need to wear masks.
Masks will not be mandatory in indoor venues which have other safety measures in place, including:
- Eat-in restaurants
- Hairdressers and salons
- Gyms and leisure centres
- Cinemas, concert halls and theatres
- Visitor attractions like museums
The government said evidence suggests that when used correctly, face coverings may reduce the chance of infecting others.
The government has been accused of mixed messaging over wearing masks in takeaways, with trade bodies and MPs saying there had been confusion about how the rules would apply.
A Department of Health spokesman said: "If you are in a premises where you are able to sit down and consume food or drink that you have bought, then you can remove your face covering in order to eat and drink on site."
The criticism came after ministers and Boris Johnson's official spokesman contradicted each other over how the rules would apply to takeaways and sandwich shops.
Speaking before the guidance was published, Kate Nicholls, chief executive of UK Hospitality, said the messaging from the government had been contradictory and it was "very late in the day" for the guidance to be confirmed.
Unison official Jon Richards also said the UK "was late to the table on face coverings and now people don't know what they should do".
He said the rules on face coverings were in place for shops and public transport but not for some other enclosed spaces such as libraries, register offices and civic centres. "The public needs clarity to end the muddle," he said.
According to the government, face coverings should cover the mouth and nose and can be as simple as a scarf or bandana that securely fits around the side of the face without having to be held in place.
The requirement to wear face coverings at transport hubs - railway and bus stations, airports and maritime ports - only applies for those areas which are fully indoors and enclosed.
You are allowed to remove a face covering in certain situations, for example to prove identification in banks or when buying age restricted products.