Spain's stock market regulator said on Wednesday that it had opened an investigation into whether social media site X, formerly Twitter, flouted rules on cryptocurrency advertising.

Carrying such allegedly deceptive advertisements would violate regulations adopted in March requiring websites, media outlets, and social networks to better vet marketing for financial services.

"In recent days we have detected advertisements on X by a non-authorised financial company," head of the CNMV regulator Rodrigo Buenaventura said in a speech to Spanish financiers.

The ads "illegally used the image of certain Spanish actors and copied the logos and brands of national media to obtain data and money from investors," he said. The advertisements in question promoted crypto currencies, according to a CNMV spokesperson.

Websites and platforms have the "obligation to verify that advertisers are not on lists of illegal companies" and that they are authorised to offer these services, Buenaventura said.

According to the regulations adopted in March, X could face a fine of at least 5 million euros ($5.35 million).