CME Group Inc. (CME) said it will press on with a planned change Monday to the way it calculates closing prices for its benchmark grain markets, despite a last-minute legal challenge from floor traders who fear the move will hurt their business.

The change will take into account both floor-based trading as well as activity on CME's electronic trading system to formulate the price at which corn, soybean and wheat contracts settle at the end of each trading session.

A group of traders on Friday filed a lawsuit in Illinois' Cook County Circuit Court seeking to block the move, which arrives as CME has broadened trading hours for its agricultural markets in response to competition from rival electronic markets.

"We believe the new settlement methodology accurately captures contract value across both the open outcry and electronic trading venues during the settlement period and is fully consistent with our functions as a designated contract market and a derivatives clearing organization," said a spokesman for CME in a statement Monday. He declined to comment on the litigation.

Shares in CME recently were off 1.4% at $268.42.

(END) Dow Jones Newswires

June 25, 2012 10:14 ET (14:14 GMT)