The Zonal Councils are the statutory (and not the constitutional) bodies.  

• They are established by an Act of the Parliament, that is, States Reorganisation Act of 1956.  
• The act divided the country into five zones (Northern, Central, Eastern, Western and Southern) and provided a zonal council for each zone.  
• While forming these zones, several factors have been taken into account which includes: the natural divisions of the country, the river systems and means of communication, the cultural and linguistic affinity and the requirements of economic development, security and law and order. 

Source: Laxmikanth