Joining the chorus of regulators calling for a crackdown on cryptocurrencies, the French and German finance ministers said digital tokens “could pose substantial risks for investors” and potentially long-term financial stability.

In a letter to fellow G20 finance ministers – signed by French Finance Minister Bruno le Maire and his interim German counterpart Peter Altmaier, along with the heads of the two countries’ central banks – they said that cryptocurrencies currently have “limited” implications for global financial stability.

“Given the fast increase in the capitalization of tokens and the emergence of new financial instruments” based on them, “these developments should be closely monitored,” the ministers said.

Cryptocurrencies “are currently largely mislabeled as ‘currencies’ in the media and on the internet,” they said, adding that a “lack of clarity” about the nature of tokens “can only fuel speculation.”

They also called for greater protections for retail investors speculating in crypto, saying “the buildup of individual exposures to such volatile tokens could have damaging consequences for misinformed investors who do not understand the risks they are exposing themselves to.”

This week, warnings have been voiced by regulators and watchdogs from the Bank for International Settlements (BIS), the European Central Bank (ECB), and Hong Kong’s Securities and Futures Commission.

ECB board member Yves Mersch said on Thursday that “cryptocurrencies” are “not money, nor will they be for the foreseeable future.”

Bitcoin is “a combination of a bubble, a Ponzi scheme and an environmental disaster,” Agustin Carstens, the head of the BIS, said.

He called on central banks to clamp down on bitcoin and other cryptocurrencies to stop them “piggybacking” on mainstream institutions and becoming a “threat to financial stability.”

RT