The US Securities and Exchange Commission has proposed a revised set of rules governing how US stocks are traded, strengthening a requirement that brokers obtain the best possible price for orders despite fears it could slow down trade execution.

The SEC voted by a margin of four-to-one to open the revised proposal on its so called Regulation NMS to public comment for 30 days. If adopted the proposals would lead to the biggest overhaul of the US stock market system in over 30 years.