In a complaint filed on Friday in the U.S. District Court in Chicago, William Braman, Mark Mendelson and John Simms said CME and its Chicago Board of Trade unit have since 2007 given high-frequency traders early access to buy and sell orders.

They said this deprived other investors of the transparent, real-time data on futures and interest rate contracts that they thought they were getting, and were paying for.

"The defendants have perpetrated a fraud on the marketplace and intentionally concealed the activities of a select class of market participants from the rest of the defendants' customers and marketplace users," the complaint said.

CME, which is based in Chicago, has denied wrongdoing.

High-frequency traders use computer algorithms to obtain split-second advantages when placing trades, and are responsible for more than half of all U.S. trading volume.

Proponents say such trading improves market liquidity, while critics say it can destabilize markets and mask manipulation.

The trading is being probed by the U.S. Department of Justice and other federal and state investigators, and came under increased scrutiny with the recent publication of Michael Lewis' book "Flash Boys: A Wall Street Revolt."

CME in a statement said Friday's lawsuit was "devoid of any facts supporting the allegations and, even worse, demonstrates a fundamental misunderstanding of how our markets operate."

It added: "The case is without merit, and we intend to defend ourselves vigorously."

Tamara de Silva, a lawyer for the plaintiffs, said in a phone interview: "Exchanges have represented to regulators and to the broad public that information isn't distorted.... I'm making allegations that the exchange allowed these practices and profited from them." She declined to provide specifics.

The lawsuit seeks class-action status for customers in financial futures contracts, and agricultural, energy, metal, equity index, foreign exchange and interest rate futures and options contracts. It seeks money damages, and a halt to alleged favoritism.

CME on March 31 prevailed in a separate lawsuit in an Illinois state court brought by floor traders who sought to overturn rules that factor in electronic trades for settling end-of-the-day grain futures prices.

The same week, high-speed trading firm Virtu Financial Inc. delayed its initial public offering. It had revealed on March 10 that it had just one losing trading day out of 1,238 such days in the five years ending Dec. 31, 2013.