SEATTLE — A federal lawsuit filed Wednesday challenges Seattle’s adoption of what would be the nation’s highest minimum wage as unfair to small franchises.

The Seattle City Council voted unanimously this month to gradually raise the minimum wage to $15 an hour. The plan gives businesses with more than 500 employees nationally at least three years to phase in the increase – four if they provide health insurance. Smaller employers get seven years.

In a lawsuit filed in U.S. District Court in Seattle, the International Franchise Association, a Washington, D.C.-based business group, said the ordinance “unfairly and irrationally discriminates against interstate commerce generally, and small businesses that operate under the franchise business model specifically.”

For example, an independently owned Holiday Inn Express in Seattle with 28 workers is considered a “large” business under the law, because Holiday Inn franchises nationally have more than 500 workers, the lawsuit argues. Meanwhile, other Seattle companies with up to 500 workers are considered “small” and given extra time to adopt the wage.

“The ordinance will impose significantly higher labor costs on small franchisees than on their non-franchised competitors,” it said. “It is foreseeable that some small franchisees in Seattle will not survive this prolonged period of unfair competition.”

But Seattle Mayor Ed Murray rejected any assertion of unfairness, saying that the franchises have advantages unavailable to other local businesses.

For example, fast food franchises have menus, a supply chain, training and advertising provided by national corporate entities, he noted.