Pornhub says the number of UK visitors to its website is down 77% compared with July, when more rigorous age checks for sexually explicit sites were introduced under the Online Safety Act.
It claims sites that are ignoring the new requirements are benefiting.
The BBC has not been able to independently verify Pornhub's claim - however, data from Google shows searches for the site have decreased by almost half since the law came into effect.
This could be a consequence of people reducing their porn use but could also be partly explained by people visiting the site through alternative means such as a VPN, which masks a user's location.
Pornhub is the most visited porn site in the world - and the 19th most visited on the entire web, according to data from Similarweb.
