IEX Group, the start-up US trading venue that has drawn fire from rivals over its attempts to become a fully-fledged stock exchange, has called on buyside firms to petition regulators to back the move in what it has called a "David vs Goliath fight".

IEX sent a note - seen by Financial News - to its buyside users on December 10 in which it encouraged them to write to the US Securities and Exchange Commission in support of its exchange application.