By Foo Yun Chee

BRUSSELS (Reuters) -Adult content platforms Pornhub, Stripchat, XNXX and XVideos will be investigated for failing to comply with rules to protect children, EU regulators said on Tuesday, in a move that could lead to fines of as much as 6% of their global annual turnover.

The European Commission said the companies had not complied with the Digital Services Act, which requires them to do more to tackle illegal and harmful content on their platforms following their designation as very large online platforms in 2023.

They also breached rules obliging companies to take risk assessment and mitigation measures of the negative effects on the rights of children and to prevent them from accessing adult content via age verification tools, it said.

"The online space should be a safe environment for children to learn and connect. Our priority is to protect minors and allow them to navigate safely online," EU tech chief Henna Virkkunen said in a statement.

Pornhub owner and Cypriot group Aylo Freesites Ltd said its sites were RTA- (restricted to adults) compliant, as rated by the Association of Sites Advocating Child Protection, and were strictly reserved for those of legal age.

"We believe that the real solution for protecting minors and adults alike is to verify users’ ages at the point of access - the users’ devices - and for websites to deny or permit access to age-restricted materials based on that verification," Aylo said in an email.

Stripchat, which is a subsidiary of Cypriot company Technius Ltd, did not respond to an emailed request. XNXX is owned by Czech company NKL Associates and XVideos is part of WebGroup Czech Republic. It was not immediately possible to reach those two companies.

The Commission said it would drop its designation of Stripchat as a very large online platform in four months' time after its average monthly number of users fell below the DSA user threshold.

Separately, the EU executive said EU countries would coordinate actions against smaller pornographic platforms to protect children.

(Reporting by Foo Yun Chee;Editing by Alison Williams)