In a speech in Dublin on Friday, EU Internal Market Commissioner Charlie McCreevy claimed MiFID, which will be reviewed at the end of the year, has achieved its objective of stimulating competition among trading venues in Europe.

Chi-X, Turquoise and Bats, among others, have all emerged in the directive's wake to challenge traditional exchanges such as the LSE and Deutsche Borse.

However, McCreevy says there also seems to have been a "significant migration" of trading to unregulated over-the-counter broker dealer venues where dark pools of liquidity have built up.

"This gives rise to questions as to whether there are unfair commercial advantages for the operators of these venues and whether the trend undermines price discovery, market integrity and efficiency for the market as a whole," says McCreevy.

Dark pools have come under increasing scrutiny from regulators in recent months, with both the Securities and Exchange Commission in the US and the UK's FSA looking into their impact on market transparency and integrity.