BRUSSELS, July 12 (Reuters) - Elon Musk's social media company X breached EU online content rules, EU tech regulators ruled on Friday in a finding that could lead to a fine and significant changes in how it operates.

The charges by the European Commission, the first issued under the Digital Services Act (DSA), follow a seven-month long investigation. The new rules require very large online platforms and search engines to do more to tackle illegal content and risks to public security.

The EU executive's preliminary findings or charges sent to X targeted the company's dark patterns, advertising transparency and data access for researchers.

It said X's verified accounts which carry a blue checkmark do not correspond to industry practice and negatively affect users' ability to make free and informed decisions about the authenticity of the accounts they interact with.

X has also failed to comply with a DSA requirement that companies provide a searchable and reliable advertisement repository, the Commission said.

X was also charged with blocking researchers from accessing its public data. The company, which will have several months to respond to the charges, could face a fine as much as 6% of its global turnover if found guilty of breaching the DSA.

"X has now the right of defence — but if our view is confirmed we will impose fines and require significant changes," EU industry chief Thierry Breton said in a statement.

Musk responded on X: "How we know you're real?"

The Commission said separate investigations continue into dissemination of illegal content on X and measures it has taken to counter disinformation.

ByteDance's TikTok, AliExpress and Meta Platforms are also being investigated under the DSA. (Reporting by Foo Yun Chee; editing by Jason Neely and David Evans)