Two MEPs who helped craft the European Union's vast trading reforms are split on how crucial data price rules should be interpreted — rules that are already causing confusion in the markets.

The EU's revised Markets in Financial Instruments Directive has created swathes of new trade data in a bid to bring more transparency to the region's markets. This data is collected by national stock exchanges and other trading venues registered under under a so-called approved publication arrangement (APA) and is supposed to be made public 15 minutes after execution.