Any city of the first class owning and operating a natural gas distribution system within such city, and owning and operating its own lateral supply line from its distribution system to a natural gas pipeline source of supply, may by ordinance, where such lateral supply line is so located with reference to any cities of the second class or villages within twenty miles of such city not then being supplied with natural gas and having no other source of gas supply available, make gas service available at retail to such municipalities and for that purpose construct, operate, and maintain connecting lines to and natural gas distribution systems in the municipalities. Such city prior to the construction of such facilities and the rendering of such service shall secure from the respective municipalities to be served a natural gas franchise as provided by law.