“Chicago may be the best sports town in the country,” FanDuel said in a news release. “So why the attorney general would tell her 13.5 million constituents they can’t play fantasy sports anymore as they know it — and make no mistake, her opinion bans all forms of fantasy sports played for money — is beyond us. Hopefully the legislature will give back to the people of Illinois the games they love.”

The opinion follows decisions in New York and Nevada to ban the sites.

Players who bet on fantasy sports assemble teams of professional athletes and compete based on those players’ statistical performances in games. By concluding that daily fantasy games constituted gambling, the New York attorney general, Eric T. Schneiderman, directed a spotlight on some professional sports leagues that have opposed gambling but maintained financial partnerships with daily fantasy sports sites.

This month, a New York judge granted the state attorney general’s injunction barring FanDuel and DraftKings from operating there. But later that day, an appeals court judge allowed the companies to keep operating until the legality of their games could be further considered.

The decision by the appellate judge, Associate Justice Paul G. Feinman of the Appellate Division of the State Supreme Court, came in an emergency hearing in which lawyers for DraftKings argued that if the company were forced to shut down even temporarily, it would be irreparably harmed. The stay by Justice Feinman will allow the companies to do business in New York until at least Jan. 4.

If the New York ruling is upheld, it will be a major setback to the daily fantasy sports industry. New York, with more than 1.2 million customers, is the largest market, and the companies will lose a combined $35 million to $40 million in revenue, according to Eilers Research. The research firm said Illinois was the third-largest market, with 6.7 percent of the customer base.