In a speech at the Sifma annual conference, SEC chairman Mary Schapiro told delegates that the regulator is drafting a proposal on sponsored access, focussing on arrangements that enable unfiltered access by non-regulated entities - in many cases, high frequency traders - to exchange systems.

"I liken it to giving your car keys to a friend who doesn't have a license and letting him drive unaccompanied," says Schapiro.

She argues that broker-dealers act as gatekeepers, maintaining the integrity of markets and that this should not be sacrificed "to give a trader a millisecond advantage".

Schapiro has also asked SEC staff to find ways to shed light on high frequency traders, who now account for more than 50% of volume.

"I believe we need a deeper understanding of the strategies and activities of high frequency traders and the potential impact on our markets and investors of so many transactions occurring so quickly. And we need to consider whether there are additional legislative authorities needed to address new types of market professionals whose activities may not be sufficiently regulated," she says.

In addition, the watchdog expects to seek public comment on co-location - the process where exchanges allow some broker-dealers to place their servers close to the matching engine of the bourse. Shapiro is worried that this offers "significant advantages" for traders who rely on speed.

The SEC has already proposed a ban on flash trading - which gives some investors a sneak peak at open order before the wider market - and last week made three specific proposals aimed at strengthening the regulation of dark pools.

Meanwhile, with the use of dark pools and high frequency trading under intense scrutiny, Goldman Sachs - which runs the Sigma-X platform - has been defending the practices to the SEC.

In a recent memo to the watchdog, the investment bank says dark pools "are a technological evolution of classic market structure that have brought benefits to institutional and retail trading alike".