France’s markets regulator has fined high-frequency trading firm Virtu Financial and exchange group Euronext €5 million each for breaking trading rules of the Paris stock exchange in 2009.

It is the first penalty imposed on a European exchange in connection with high-speed trading and comes after years of heated debate into whether stock markets are slanted in favour of sophisticated traders which use computers to trade in fractions of a second. A divisive practice, regulators are still unsure whether high-frequency trading benefits markets by adding liquidity or harms them by distorting price discovery.