In a significant development, Citadel Securities has sued the Securities and Exchange Commission (SEC) over its decision to approve a new mechanism for trading stocks at IEX Group Inc.

“The SEC failed to properly consider the costs and burdens imposed by this proposal that will undermine the reliability of our markets and harm tens of millions of retail investors,” said a spokeswoman for Citadel Securities.

The development marks an increase in Citadel Securities’ dispute over IEX’s “D-Limit” order type.

The D-Limit is designed to provide traders a way trade in stocks at the exchange while protecting them against unfavorable price moves.

Earlier, Citadel Securities had asked the SEC to reject IEX’s proposal saying the D-Limit will damage the stock market’s integrity. However in August, the SEC sided with IEX allowing its plan to go ahead.

According to a copy of the court filing, Citadel Securities has asked the U.S. Court of Appeals for the District of Columbia Circuit to review the SEC’s decision to approve the D-Limit order.

The SEC did not immediately respond to requests for comment.

In a statement, IEX President Ronan Ryan said he was confident the SEC’s decision will be upheld.

“Since its launch on Oct. 1, D-Limit is already proving valuable to a broad set of market participants,” said Ryan. “From our perspective, this recent action should only encourage more investors, brokers and market makers to use D-Limit given that the protections we have created are clearly working”.

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Tags: brokers, Citadel Securities, Court of Appeals, D-Limit, District of Columbia Circuit, IEX Group Inc, IEX President, integrity, investors, market, market participants, new mechanism, reliability, retail investors, Ronan Ryan, SEC, Securities and Exchange Commission, stock markets, trading stocks