Defenders of sponsored access note that so far there has been scant evidence that the arrangements pose risk to markets. They say brokerage firms employ rapid posttrade checks that can quickly shut down an operation if orders run amok. They also say high-speed traders are sophisticated enough to avoid problems. For now, a patchwork of loose rules among exchanges and brokers governs the practice.

The SEC says naked access poses threats to stability, whereas "flash orders," a type of high-speed trading that recently has caught the attention of politicians, raise concerns of fairness. Flash involves some traders getting a sneak peek at market orders before other investors. The SEC has recently sought to ban flash orders.

Sponsored access has spread in recent years along with high-speed trading. On the Nasdaq Stock Market, the biggest volume provider is Los Angeles-based Wedbush Securities, a closely held brokerage firm with more than $2 billion in assets, according to the Nasdaq. Wedbush says more than 75% of the volume in its clearing unit stems from traders using sponsored access.

Goldman Sachs Group Inc. and Credit Suisse Group AG provide a variant of sponsored access, according to representatives at the firms.