Two decades after New Haven’s English Station power plant stopped producing energy for United Illuminating, state officials have ordered the owners to conduct a massive clean-up of the property, which is contaminated with hazardous PCBs, or polychlorinated biphenyls. “English Station has been a potential source of pollution to Fair Haven and the waters of the state for too long. It must be cleaned up by all those responsible for its present condition,’’ said Attorney General George Jepsen, whose office is working with the state Department of Energy and Environmental Protection (DEEP). The administrative order announced Thursday requires that the current and previous owners of the plant make a full investigation of the contamination on, and emanating from, the site; submit a remediation plan for DEEP approval that is in compliance with federal and state laws and regulations; and then remediate the site in accordance with the approved plan. The property, on the Mill River, contains the former electric-generating plant and a warehouse. The parties named in the order include the current owners, Asnat Realty, LLC of Bayside, N.Y. and Evergreen Power, LLC, of Wilmington, Md., as well as Quinnipiac Energy, LLC; Grant Mackay Demolition; and the United Illuminating Company, which previously owned the site. The plant is shut down, and access to the property has been limited, pending submission of a formal plan to clean up extensive contamination by PCBs, a known carcinogen, as well as heavy metals and other contaminants.