In an official bulletin, the CNMV said the US$5.8-million fine was imposed for  “a very serious ongoing violation”.

The regulator had launched an investigation in November 2023 after detecting advertisements on X promoted by an unauthorised financial company, Quantum AI, which was marketing cryptocurrencies.

The postings violated a regulation introduced in March 2023 requiring websites, media outlets, and social networks to better control advertising of financial products, the CNMV said.

Under the rules, online platforms in Spain are legally required to verify that advertisers are not on lists of illegal companies and that they are authorised to offer financial services, Rodrigo Buenaventura, the CNMV president at the time, said in November 2023 when announcing the probe.