The rise of so-called dark pools and other off-exchange strategies aimed at large banks and institutional traders comes as regulators on both sides of the Atlantic grapple with balancing the market efficiencies the alternative venues say they generate with the impact on individual investors.

Private venues are seen as a more efficient way for transacting large chunks of shares, but critics worry that if so much trading is done privately, publicly available prices set by exchanges will become less accurate. Dark pools are electronic platforms designed for institutions to carry out major stock trades anonymously.