MGM Resorts has kept its promise to block Mashantucket Pequots and the Mohegan Sun’s proposed “box of slots” in Connecticut.

MGM Resorts International filed a lawsuit Tuesday in U.S. District Court in New Haven with Gov. Dannel Malloy as a defendant, saying a law he signed that could lead to a third tribal casino in the state is unconstitutional because it gives two tribes “preferential treatment,” barring out-of-state competitors from trying to develop a casino in Connecticut.

Malloy has approved a legislation in June authorizing the tribes to build a jointly run $300m casino along Interstate 91, to compete with MGM Resorts’ $800m casino just across the border in Springfield, Massachusetts. The casino is expected to open in 2018.

MGM Resorts International President Bill Hornbuckle claimed that it has applied for a casino license in the state after the passage of the act but was rejected last week.