By Michelle Roberts
Health editor, BBC News online
Face coverings are now compulsory in many indoor spaces in England, as stricter measures are being introduced to tackle the Omicron variant.
Similar rules have already been introduced in other parts of the UK.
Face coverings are now required in most public indoor venues, such as cinemas, theatres and places of worship.
The government previously reintroduced masks in shops, on public transport and at hubs like railway stations.
Masks are also compulsory for staff and customers in:
Under-12s are exempt, as are those who can't wear face coverings for medical reasons.
Secondary school pupils are "strongly advised" to wear masks in communal areas, as are staff and visitors.
The government says face masks will not be required in pubs, restaurants or gyms.
The reasoning is that it would not be practical to wear while eating, drinking, or exercising.
You can also take off a mask to sing, when it is "reasonably necessary" – for example, if you are in a choir or on stage.
In England, Transport for London (TfL) officers and police can fine people £200 for their first offence. This drops to £100 if paid within 14 days.
The penalty doubles for each subsequent offence, with no discount for quick payment. So, a second penalty would be £400, a third £800, up to a maximum of £6,400.
In Wales, the penalty for a first offence is £60, with the fine doubling for each subsequent offence – up to a maximum of £1,920. Repeat offenders can also be prosecuted in court where fines are unlimited.
In Scotland, £30 fines are issued to people who don't wear face coverings in public buildings, rising to almost £1,000 for repeat offenders.
The British Retail Consortium, which represents shops across the UK, says enforcement is a matter for the police, but that customers should respect the rules and be considerate.
The Association of Convenience Stores, representing 33,500 local shops, says it doesn't expect staff to challenge unmasked customers as it can be "a major trigger for abuse".
Police will enforce regulations "where it is necessary," says the National Police Chiefs' Council (NPCC).
"If officers encounter individuals not wearing a face covering on public transport or in shops, they will engage with them, explain the risks and encourage them to comply."
According to the NPCC, 2,306 Fixed Penalty Notices were issued in England and Wales in the year to July. Meanwhile, 641 people were fined for failing to wear a mask on public transport or for obstructing those enforcing the law.
In October, TfL said it had prosecuted 1,733 passengers since June 2020 for failing to wear a mask – resulting in an average fine of £549.
In Scotland, masks must be worn in most indoor public spaces including public transport, shops and gyms. They must be worn in pubs and restaurants when not seated, and in the workplace. They are compulsory for all school staff as well as secondary school pupils and are required in indoor public spaces in universities.
In Northern Ireland, post-primary pupils must wear face coverings inside school buildings, as must staff if they can't socially distance, and masks must also be worn on public transport, in shops and hospitality venues.
In Wales, masks are legally required on public transport. Customers are strongly advised to wear them in pubs and restaurants when not eating or drinking. Secondary school pupils are asked to wear masks in class.
Evidence suggests transmission mainly happens indoors where people are close together.
Covering the nose and mouth reduces the spread of coronavirus droplets from coughs, sneezes and while speaking.
The main purpose is to protect others although there is some evidence they offer protection to wearers.
Standard surgical masks are acceptable. It is also possible to buy FFP2 and FFP3 masks used by healthcare workers which offer higher protection. However, these must be fitted correctly to work.
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By Michelle Roberts