WASHINGTON—The Biden administration has sued to block the merger of two of the world’s largest insurance brokers, asserting the deal could eliminate competition, raise prices and hamper innovation for U.S. businesses, employers, and unions that use the companies’ services. The Justice Department on Wednesday announced the antitrust suit filed in federal court in Washington seeking to stop AON’s proposed $30 billion acquisition of rival benefits and risk consultant Willis Towers Watson. Justice Department officials said the proposed merger would bring together two of the “Big Three” global insurance brokers—the third is Marsh McLennan—and eliminate competition in five markets. They are property, casualty and financial risk, health benefits, actuarial services for certain pension plans, retiree insurance exchanges, and reinsurance. The companies provide guidance to many major U.S. companies on administering health and retirement benefits, with the aim of keeping costs down by managing risk. From the tone of their joint statement …