Whenever in this state a city of the metropolitan class and one or more adjacent municipalities, sanitary and improvement districts, or unincorporated areas are served in whole or in part by a common public utilities system, owned and controlled by a single corporate public entity as provided for in sections 14-2101 to 14-2157, then the territory within the limits of the city of the metropolitan class and such adjacent municipalities, sanitary and improvement districts, or unincorporated areas, including any sanitary and improvement district or unincorporated area without the city of the metropolitan class or adjacent municipalities that may be now or hereafter served in whole or in part by the common public utilities system, shall form and constitute a public utilities district, except as provided in this section, to be known as the Metropolitan Utilities District of ....................... (inserting the name of the city of the metropolitan class). A municipality, not of the metropolitan class, now actually operating a general waterworks system of its own, shall not be included in the utilities district so long as it continues to operate its own water plant. No sanitary and improvement district or unincorporated area without the adjacent municipalities shall become a part of the utilities district except upon formal approval and proclamation by the board of directors.