Full guidance on wearing face coverings in shops in England has been released, less than 12 hours before the new rules come into force.
Coverings will be mandatory in enclosed public spaces including supermarkets, indoor shopping centres, transport hubs, banks and post offices.
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.
There are exemptions 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.
Police will have powers to enforce the rules, although this will be a last resort and officers will not be patrolling premises.
Masks will not be mandatory in indoor venues which have other safety measures in place, including: