Cineworld has written to its customers as Plan B omicron rules come into force across England on Friday (December 10).
It has said that face coverings are now mandatory in cinemas for all staff and customers, except while eating or drinking, or if you are medically exempt.
Previously, it was up to people's personal sense of responsibility to wear masks or not.
Read more: Plan C measures being drawn up for tougher Covid rules in England, say reports
However, it has said that customers in England will not need to provide evidence of their Covid status when visiting its cinemas.
The chain wrote to all customers on Thursday evening (December 9), on the eve of changes announced by Prime Minister Boris Johnson earlier this week.
It added that it continues to operate enhanced cleaning in all areas within its cinemas and hand sanitising stations are available at all sites.
As part of the Government's Plan B measures, people are also told to work from home from Monday (December 13) where possible and there are new rules for visits to care homes.
There are also reports about Plan C which could see making vaccine passports mandatory for pubs and restaurants and making face masks compulsory.
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