Investment Technology Group, the agency broker, is stopping sellside algorithmic access to its crossing network in order to increase the average size of executed block trades, which has fallen as the number of electronic trading venues has increased.

The number of dark liquidity pools, which do not publicly display prices, has doubled to more than 40 since the start of last year, according to market estimates. Many are crossing networks of one form or another that match buyers and sellers anonymously, minimising information leakage and market impact, often with lower direct trading costs.