The European Union's top markets regulator has joined the chorus of voices raising concerns over initial coin offerings, echoing recent warnings from US authorities that the firms issuing them risk breaking the rules.

The use of ICOs has undergone rapid, global growth — more than $2bn has been raised through coin offerings this year, according to data from research firm Autonomous. In these transactions, a startup sells tokens, often in exchange for cryptocurrencies such as bitcoin. But possessing the tokens does not confer ownership or economic rights, unlike buying company shares.