Ofcom is investigating four companies, operating a total of 34 porn sites, over whether they are complying with its new age check requirements.

The regulator said on Friday more than 6,000 sites allowing pornography and other adult content would start using “highly effective” tools to verify or estimate whether users were over or under the age of 18.

But Ofcom says some sites may be ignoring its new rules – designed to stop children stumbling across porn or other content deemed harmful by lawmakers.

It has opened formal probes into 8579 LLC, AVS Group Ltd, Kick Online Entertainment S.A. and Trendio Ltd, which it says have more than nine million monthly visitors combined across their sites.

“These companies have been prioritised based on the risk of harm posed by the services they operate and their user numbers,” Ofcom said in a press release on Thursday.

The regulator is already investigating a range of platforms over similar suspicions.

It previously opened probes into online message board 4chan and porn provider First Time Videos LLC in June.

Prior to that it launched formal investigations into an online suicide forum, file-sharing services and a company that runs a so-called “nudifying” site.

“We expect to make further enforcement announcements in the coming weeks and months,” the regulator said.

Technology Secretary Peter Kyle welcomed the announcement and said it “goes to the very heart of what the Online Safety Act is here to do”.

“These laws have nothing to do with censorship or policing adults seeking to access legal content,” he said.

“Those who suggest otherwise are playing politics with child safety and have no practical alternatives for protecting our children from content they should never see – content that can cause lasting, even fatal, damage.”

Since age verification requirements took effect on Friday, people in the UK have been asked to confirm their age across a range of different platforms including social media platforms Reddit and X.

Spotify confirmed to the BBC on Wednesday that it may prompt people in the UK, among other locations including Australia, to confirm their age if accessing music videos rated 18+ by uploaders.

The sweeping rules have prompted a backlash from some who see the requirements as an overreach or potential privacy concern.

More than 400,000 people have now signed a petition calling for the repeal of the Online Safety Act – the law requiring age checks to stop children encountering adult content.

The government said in response it has no plans to withdraw the law.

Some have also criticised the effectiveness of Ofcom’s implementation of UK age check requirements, and their potential circumvention using virtual private networks.

Apps providing free VPNs – which allow people to disguise their location online, in order to access the web from a different location – topped free app download charts on Apple’s App Store after the rules came into effect on Friday.

Ofcom has defended the measures as no “silver bullet” but said they will help make the internet safer for children by reducing the chance of them “stumbling across porn”.