United and American Airlines have filed a lawsuit against the city of Chicago to stop the second phase of an expansion of O’Hare International Airport.

In a letter sent to Chicago Mayor Richard Daley on Friday, United Airlines along with American Airlines threatened legal action against the city. Today, the airlines followed through with that filing a lawsuit with the Cook County Circuit Court.

The companies stated they are “alarmed at the City’s actions to proceed with the issuance of debt for this project without airline approval.” The airlines say the lease agreement with O’Hare gives the companies voting rights on capital projects, and the city has violated that right.

The companies say continuing with the second phase of the O’Hare Modernization Program now would make O’Hare among the highest costing airports in the country restricting the airlines’ growth.

The city is depending on a portion of airline revenue to help pay for the airport expansion.

In a statement the City of Chicago says it continues to move forward to secure funding for the expansion and believes it has the legal rights.

“We remain willing to discuss modernizing O’Hare with the airlines that serve the airport; however, timing is essential. The opportunity to realize the OMP’s (O’Hare Modernization Program) benefits for the region and national aviation system cannot be lost.”