CME Group Inc was sued last week by brokers and traders in a last-minute effort to halt new procedures to settle end-of-day prices for agricultural futures in its Chicago Board of Trade, which plaintiffs say could put them out of business.

The lawsuit was filed in Cook County court by a group of 24 traders and brokers, seeking to block any new rules that would diminish or eliminate the role of pit traders in determining settlement prices.

In May, CME and subsidiary CBOT said they would change the current price-settlement system to a blended model, using an algorithm that accounts both for floor-trading activity and electronic-trading information, given the increase in volume of electronic futures trading.

The move to a blended plan, initially announced in December, met with significant opposition from floor traders. After being delayed several times, a June 25 implementation date was set.

In the complaint, plaintiffs say the move will cause "a rapid, dramatic decrease in trades being sent to the plaintiffs in the 'pits,' with the result that they, and the overwhelming majority of other floor traders, will lose business immediately and will have to close their operations forever."

The change could also eliminate hundreds of jobs for traders and their support staff, including clerks, runners and others, the complaint said. Some traders have banded together in groups like "Save the Floor," an organization that opposes any changes they think may threaten the survival of pit trading.

A representative for CME Group was not immediately available for comment on Friday evening. CME Group Chairman Terrence Duffy and Chief Executive Officer Phupinder Gill are also named as defendants in the suit.

Last month, CME implemented nearly non-stop electronic trading hours for grain markets, angering traders who worry that the changes will steal trading volume away from the pits.

On Monday, the open-outcry trading session will be extended by 45 minutes to help synchronize the end of electronic and pit trading, responding to criticism from some market users.

The case is McKerr et al. v. The Board of Trade of the City of Chicago Inc. et al, in the Circuit Court of Cook County, Illinois County Department, Chancery Division, no. 12-23-185