Face coverings have become mandatory in more indoor settings in England and Scotland following a recent spike in coronavirus cases.
Places where coverings must now be worn in both countries include museums, places of worship and aquariums.
Other new settings in England include cinemas and funeral homes, and in Scotland, banks and beauty salons.
Coverings will also become compulsory in all public enclosed spaces in Northern Ireland from Monday.
It comes as new quarantine rules for travellers to the UK took effect at 04:00 BST.
Anyone now arriving in England, Scotland or Northern Ireland from Belgium, Andorra and the Bahamas will have to quarantine for two weeks. The same rules have already been put in place in Wales.
Quarantines have already been re-imposed for passengers from Spain and Luxembourg.
Lockdown measures were reintroduced in Preston from midnight after a rise in Covid-19 cases. Residents in the Lancashire city are facing stricter restrictions, which include banning separate households from meeting each other at home.
The move brings Preston in line with measures in east Lancashire, Greater Manchester and parts of west Yorkshire. The number of cases in Preston has jumped sharply in the past week, and almost half are among people aged 30 and under.
Prime Minister Boris Johnson announced the changes to the rules on face coverings in England last week when he also postponed the further easing of lockdown due to the spike in virus cases.
Those who break the law on wearing masks in England face fines of up to £100, with the PM saying there would be a "greater police presence" to ensure the rules were being followed.
Scotland's First Minister, Nicola Sturgeon, announced the new face mask rules, along with other measures, on Friday, explaining that the "risks are heightened" as the country eases lockdown restrictions.
In Wales and Northern Ireland it is only currently compulsory to wear a face covering on public transport.
Toby Bradon, general manager for Vue cinemas in the UK, Ireland and Denmark, said wearing face coverings had become "the new normal" but customers would be able to remove their masks to eat and drink in its cinemas.
He added: "We recognise that some people will be exempt so it is appropriate that we encourage people to wear them but we won't be policing it."
London's Science Museum said while customers would be required to wear face masks in line with government guidance when it reopens on 19 August, it hoped visitors would be "respectful" of those who were exempt from doing so.
Those exempt from wearing face coverings include children under 11, those with a physical or mental illness or impairment or disability and employees in indoor settings.
People are also able to remove their mask if asked to do so in a bank for identification or in order to receive treatment or services, such as a haircut.
Under the government rules face coverings are not required for those getting married or at restaurants, bars and pubs with table service.
Speaking to the BBC on Saturday, Schools Minister Nick Gibb said pupils over 11 years of age will be expected to wear masks.
But he reiterated they will not be necessary in the classrooms as "there are a whole raft of other measures we've introduced in schools to make sure we minimise the risk of transmission".
"The advice we've had about wearing masks in school is if they're not competently handled you can actually increase the risk of spreading the virus," he said.
In Scotland, the expanded list of settings where masks are required was announced alongside the tightening of other anti-coronavirus measures, particularly for the hospitality sector.
From Friday 14 August the recording of contact details for customers of pubs and other venues will become mandatory rather than advisory.
Businesses should also be asking people to pre-book tables and there should be no queues.
On Saturday, the UK reported a further 55 people had died with the virus, taking the total to 46,566. A further 758 people have tested positive for Covid-19 in the past 24 hours, according to the latest government statistics.
On Friday, the latest reproduction number - the R number - for the UK was published and was estimated to be between 0.8 and 1.0.
However, the government's Scientific Advisory Group for Emergencies (Sage) said it did "not have confidence" that the R number in England was below one.
R describes the number of people that one infected person will pass the virus on to, on average. A value above 1.0 means cases are growing exponentially, whereas if the number is kept lower than 1.0 the disease will eventually peter out.
But it came as data from the Office for National Statistics suggested the number of people testing positive for coronavirus may be levelling off in England.
In another development, the Department for Transport has announced that emergency funding for bus and tram operators in England is being extended. The money will support services in September when schools are set to fully reopen and brings the total amount spent during the pandemic on the sector to at least £700m.
Where do I have to wear a face covering in England?
- public transport
- transport hubs, like airports
- shops and supermarkets
- shopping centres
- auction houses
- premises providing legal or financial services, like banks or post offices
- personal care and beauty premises, like hair salons and tattoo parlours
- attractions, like museums and theatres
- libraries and public reading rooms
- places of worship
- funeral service providers
- community centres and social clubs
- public areas in hotels and hostels
- storage and distribution facilities
- NHS settings such as hospitals
People should also wear face coverings at any indoor places not listed but where social distancing may be difficult or you will come into contact with people you do not normally meet.
Where do I have to wear a face covering in Scotland?
- shops and supermarkets
- public transport
- takeaway restaurants
- estate agents
- beauty salons
- indoor zoos
- visitor farms
- any other indoor tourist, heritage or cultural site
- banks, building societies, credit unions
- community centres
- post offices