Chicago-based Niro Law Ltd., which is known for representing nonpracticing entities in patent litigation, is trading accusations with a U.K.-based litigation financier that helped fuel some of the firm’s cases.

In a federal court complaint filed Wednesday in Chicago, U.K. litigation funder 1st Class Legal Ltd. alleged that Niro breached a contract from late 2011 that provided the law firm about $3.54 million in funding for multiple patent infringement lawsuits.