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 were earlier re-imposed for passengers from Spain and Luxembourg.
Read more: BBC News