Am Law 200 firm Goulston & Storrs is facing a $300 million lawsuit based on allegations the firm failed to disclose client conflicts and relevant information to a construction client in Boston, ultimately causing a major real estate development project to flounder. The firm says the claims are without merit.

The complaint, filed Friday by construction magnate John Fish and his company JFF Cecilia in Massachusetts State Court, centers on a billion-dollar, mixed-use high rise project in Boston’s Back Bay neighborhood, referred to in the complaint as the St. Cecilia’s Project. Also involved in that project was Stephen Weiner and his firm, Weiner Ventures, which conceived the project, according to Fish’s complaint.