MADRID (Reuters) -Spain's stock market supervisor has fined Elon Musk-owned social media platform X 5 million euros ($5.8 million) for failing to make sure that a cryptoasset company that used X for advertisements had authorisation to provide investment services.

The fine, dated November 3, was published in Spain's official bulletin on Thursday.

X, formerly known as Twitter, did not immediately reply to an emailed request for comment. The fine can be appealed before Spain's High Court.

Madrid took steps to regulate the rampant advertising of cryptoassets in 2022, tasking the CNMV watchdog with vetting mass campaigns and making sure investors were aware of the risks involved.

"(CNMV has decided to) impose a fine on Twitter International Unlimited Company ... for failing to fulfil its duties to verify whether Quantum AI was authorised to provide investment services by the CNMV and whether Quantum AI was included in the list of entities warned about by the CNMV or by foreign supervisory bodies," the document said.

($1 = 0.8575 euros)

(Reporting by Emma Pinedo; Edititng by Andrei Khalip and Emelia Sithole-Matarise)