Whereas, at the end of 2012, Tradebook Europe executed just under 10% of its traded value in the dark, over the last eight weeks this percentage has jumped to more than 15%, says Chris Heeringa, product manager and global market structure specialist at the firm.

While some of this relates to Tradebook's recent extension of dark pool coverage to two new venues, a deeper analysis of client trading data indicates that more liquidity is consistently available in the various non-displayed trading venues, remarks Heeringa.

Taking figures from the firm's BSmart algorithm, which posts to lit and dark pools, Tradebook observed a shift to dark execution in UK, German and French stocks during the second half of 2012, moving from 14% of traded value in H1 2012 up to 20% by year end.

During the first months of 2013, the trend has accelerated, says Heeringa, with the dark share continuing its rise to 23%.

Across the markets as a whole, the current share of dark pools is estimated to be somewhere around seven percent, according to figures from Rosenblatt, with a large fraction of market participants using them for limited price check sweeps before aggressive orders are sent to the lit market.

"Of course, the lit markets still represent the majority of traded volumes," accepts Heeringa. "The question is, what are the potential costs for accessing this, especially for large orders? And, if the observed trends continue, it is just matter of time - short, I believe - before the majority can no longer afford to limit their dark pool access to just sweeping for price improvement; they will have to start using dark pools more actively to tap into available liquidity at all stages of the order."