BRUSSELS, March 26 (Reuters) - Adult content platforms Pornhub, Stripchat, XNXX and XVideos have been charged with breaching EU rules by letting children access pornographic content on their sites, EU regulators said on Thursday, which could lead to hefty fines.

The charges follow a 10-month-long investigation under the bloc's Digital Services Act, which requires large online platforms to do more to tackle illegal and harmful content.

"Children are accessing adult content at increasingly younger ages and these platforms must put in place robust, privacy-preserving and effective measures to keep minors off their services," EU tech chief Henna Virkkunen said in a statement.

The European Commission, charged with enforcing the act, said the companies did not use objective and thorough methodologies to assess the risks to children accessing their services.

It accused Pornhub, owned by Cypriot group Aylo Freesites, Stripchat, a subsidiary of Cyprus's Technius, XNXX, owned by Czech group NKL Associates, and WebGroup Czech Republic unit XVideos of being more worried about their reputations than societal risks to minors.

The regulator also took issue with the companies' self-declaration tool which allows users to access their platforms with a simple click confirming they are over 18.

It said this and measures such as page blurring and content warnings were not effective to stop children from accessing their sites.

The Commission said that Pornhub, Stripchat, XNXX and XVideos need to implement privacy preserving age verification measures to protect children from harmful content.

Companies risk fines as much as 6% of their global annual turnover if found guilty of Digital Services Act breaches.

"We have received the European Commission's preliminary findings and are carefully reviewing them. These are preliminary findings, not a final decision, and our detailed analysis is ongoing. We will continue to engage constructively with the Commission as we present our position," said Aylo.

"Protecting minors online is a goal we firmly share with the Commission. At the same time, our goal is to get age verification right. Our experience across multiple jurisdictions shows that current website-level age-verification solutions often fail, driving users toward unregulated sites with little or no safety infrastructure, and raising serious data privacy concerns," added Aylo.

Stripchat did not respond to Emailed request for comment and the contact details for the two Czech sites were not available.