The top European markets regulator delivered its strongest warning yet about the risks of using social media for investing, cautioning that any effort to coordinate trades online could violate market-manipulation rules.

The Paris-based European Securities and Markets Authority said it was acting following the frenzy in trading in U.S. companies including GameStop Corp. The trading, fueled by retail investors on Reddit message boards, has prompted investigations into potential market manipulation as well as calls for greater scrutiny of mobile trading apps, short-selling rules, broker solvency and social media.

“Discussing the opportunity to buy or sell the shares of an issuer does not constitute market abuse,” ESMA said on Wednesday. “However, organizing or executing coordinated strategies to trade or place orders at certain conditions and times to move a share’s price could constitute market manipulation.”

The regulator, which sets standards across the European Union, said that disseminating false or misleading information could also be against the rules.