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.
Under the OSA, anyone accessing such websites in the UK now has to prove they are over 18 with age checks such as facial identification.
The firm's claim is the latest indication that people in the UK are changing how they use the internet since the Online Safety Act came into effect.
According to Ofcom, visits to pornography sites in general in the UK have reduced by almost a third in the three months since 25 July.
The regulator said the new law was fulfilling its primary purpose of stopping children from being able to "easily stumble across porn without searching for it".
"Our new rules end the era of an age-blind internet, when many sites and apps have undertaken no meaningful checks to see if children were using their services," the watchdog said.
Ofcom told the BBC it believed the number of people using VPNs for general use reached 1.5 million daily in July, after the law came in, but has since decreased to around one million.
Meanwhile, research by Cybernews counted more than 10.7 million downloads of VPN apps in the UK from the Google Play Store and Apple App Store across 2025.
"It is likely that people not wanting to verify their age or identity to access sexual content, for example because of privacy concerns, are using VPNs to get around this," Dr Hanne Stegeman from the University of Exeter told the BBC.
"As the location of website visitors are usually determined through IP addresses, it could be that those figures are inaccurate when a portion of visitors are using VPNs."
And Cybernews information security researcher Aras Nazarovas told the BBC people in the UK "can and do" use VPNs.
"After age checks kicked in, VPN apps jumped to the top of the UK App Store, and at least one provider saw a 1,800% surge in downloads," he said.
"So part of Pornhub's 'missing' UK audience hasn't vanished - it's being reclassified as non‑UK traffic."
But he said he believed "the rest" was indeed "users shifting to sites that don't require age checks".
'Exponential growth'
Alex Kekesi, an executive at Pornhub's parent company Aylo, told the BBC the new rules were unenforceable.
She said Ofcom faced an "insurmountable task" trying to get an estimated 240,000 adult platforms - visited by eight million users per month in the UK - to follow the rules.
This compares with the regulator taking action against fewer than 70 sites for non-compliance.
Ofcom says it prioritises sites to be investigated based on how risky they are and their number of users.
And Ms Kekesi claimed some pornographic sites have benefited from flouting the rules. The BBC has not independently verified this.
"There are a number of sites whose traffic has grown exponentially, and these are sites that are not complying," she said.
Ms Kekesi also has concerns about the content on some of these sites.
She told the BBC of one which seemed to encourage users to search for content featuring girls below the age of consent.
Aylo says it has shared details of this and other sites with Ofcom.
The regulator has defended the way it enforces the new rules, saying increasing traffic to sites can be one factor that triggers an investigation.
"Sites that don't comply and put children at risk can expect to face enforcement action," it told BBC News.
Ofcom's data shows that the top 10 most popular sites all have age assurance deployed. These sites represent a quarter of all visits to adult sites from across the UK.
It adds that over three quarters of daily traffic to the top 100 most popular sites are going to sites that have age assurance.
The government has also defended the regulator, and said protecting children online was a "top priority" for ministers.
"Where evidence shows further intervention is needed to protect children, we will not hesitate to act," it added in a statement.
Should devices do the checks?
Ms Kekesi spoke to the BBC while in the UK for a meeting with Ofcom and government officials, where she has been making Pornhub's case that age checks should be done at device level, rather than by individual websites.
She said the UK stands out in having persuaded the platform to introduce age checks.
A number of jurisdictions have sought to compel Pornhub to check its users' ages, but the response of the site has been to block users rather than comply.
Ms Kekesi said the UK was different because it allowed sites to offer a range of different solutions, meaning that Pornhub could use methods - such as email-based checks - which didn't require collecting biometric data.
She denied that the threat of hefty fines for non-compliance had been the primary motive for complying, pointing to the contrast with France - its second biggest market - where it had cut off access rather than agreeing to what regulators demanded.
Ian Corby of the Age Verification Providers Association rejected calls for a switch to device-based verification.
But he added the group shared a desire for a "level playing field" meaning age checks should be "robust, not superficial or fake".
Chelsea Jarvie, a cybersecurity company founder who has been researching methods of age assurance for a PhD at Strathclyde University, told the BBC both approaches to age checks would be needed - with neither age verification on platforms nor devices being a "silver bullet".
"For somebody to truly be safe online we need different layers of controls throughout their browsing journey," she said.
