Solomon Friedman of Ethical Capital Partners (ECP), which owns Aylo, said the company believed Ofcom was "working in good faith" to enforce age check requirements.

"The problem here, however, is not the regulator - it is the law," he said.

"You have a dedicated regulator working in good faith, but unfortunately, the law they are operating under cannot possibly succeed."

Friedman said six months after the requirement for sites allowing sexually explicit content took effect in the UK, people were still able to easily access porn - such as by searching for it online.

Emma Drake, partner of online safety and privacy at law firm Bird and Bird, said research cited by Aylo suggesting adults were seeking riskier porn sites also stated overall use of porn sites by adults had fallen

"The same must be true of children," she told the BBC.

"The determined will find alternative routes, like the VPNs or the new entrants Aylo mentioned, but adding barriers to the most well-known sites can still protect a very large number of children who won't make that effort."

The company reiterated its position that device manufacturers such as Apple, Google and Microsoft were best placed to introduce technical measures to stop children accessing porn sites.

"When access is controlled at the device level, it is efficient, it's effective, it's privacy-preserving," Friedman said.

Ofcom's spokesperson said there was "nothing to stop" tech firms from coming up with age assurance methods at device level, adding "we would urge the industry to get on with that if they can evidence it is highly effective".

But they said its job was "to enforce the rules as they stand".

"We've put in place age assurance rules that are flexible and proportionate, and we have seen widespread adoption," the spokesperson added.

Cyber security expert Dr Chelsea Jarvie told the BBC controls implemented at device level, while likely to play a role in age assurance, "are not a silver bullet".

"Virtual Private Networks continue to offer a workaround which is why protecting children online requires layered controls rather than reliance on any single measure," she said.

VPNs allow people to effectively disguise their location online, and give the appearance of using the internet as though they are in a different country.

Downloads of VPN apps soared in the UK after age verification requirements took effect on 25 July.

Peers in the House of Lords recently voted, external to pass an amendment to the Children's Wellbeing and Schools Bill to "prohibit the provision" of VPNs to children.

Additional reporting by Laura Cress and Chris Vallance