Europe's trade body for stock market regulators is set to review the rules that have encouraged the emergence of a slew of new dark pools, amid concerns on both sides of the Atlantic that anonymous trading systems might be reducing market transparency.

In a report on the impact of markets in financial instruments directive, the European initiative aimed at creating a single market for financial services, the Committee of European Securities Regulators said it will review waivers in Mifid that allow dark pools to hide pre-trade prices.