The S.E.C. declined to comment on Mr. Schumer’s letter, though some officials acknowledged they were investigating the technique and expected new regulations to be issued by this fall.

Image Senator Charles E. Schumer wrote to the S.E.C. saying he intended to introduce legislation barring flash orders. Credit... Jay Mallin/Bloomberg News

When buy or sell orders are submitted to marketplaces like Nasdaq, they are sometimes flashed to a collection of high-frequency traders for just 30 milliseconds  0.03 seconds  before they are routed to everyone else. In that half-second, fast-moving computer software can gain valuable insights regarding growing or declining demand in certain stocks, and can trade ahead of other market participants, pushing prices up or down.

Although anyone can gain access to flash orders by paying a fee, they are useful only to traders who have computers powerful enough to act on the data within milliseconds. In recent years, some of the largest financial companies, including Goldman Sachs, have earned enormous profits with such computers, which are very expensive and often housed right next to the machines that power the marketplaces themselves.

While markets are supposed to ensure transparency by showing orders to everyone simultaneously, flash orders are currently allowed because of a loophole in securities regulations that allows for immediate trades.

“I’m against anything that advantages anybody over the rest of the market, and this clearly does, even though it’s momentary,” said Arthur Levitt, who headed the S.E.C. from 1993 to 2001, and today works as an adviser to Goldman Sachs and Getco L.L.C., one of the largest high-frequency trading firms.