CHICAGO — The Illinois Supreme Court on Thursday dealt another setback to Mayor Rahm Emanuel, ruling that a 2014 law that required Chicago city workers to pay more toward their retirements while also scaling back their future benefits was unconstitutional.

The court said the plan was a violation of the rights of employees, who are protected by the Illinois Constitution under a clause that states pension benefits “shall not be diminished or impaired.”

“The pension protection clause does not guarantee any particular method of funding, but, rather, guarantees the right to be paid,” the ruling said.

The decision is likely to make it more difficult for city leaders to resolve a developing financial crisis exacerbated by their obligation to pay into pension funds, which have been underfunded for years and are under increasing pressure as more workers retire and then live longer.