A referendum on the new Spanish constitution or "organic law" was held on 14 December 1966, with all Spaniards over age 21 being allowed to participate. The new constitution stipulated a slight restraint on General Francisco Franco's dictatorial powers, designated an office of "Chief of Government" independent from that of Head of State (although both were held concurrently by Franco until 1973), serving terms of five years, nominated by the Council of the Realm and approved by the Chief of State, an office which after Franco's death would be replaced by that of King of Spain with the same authorities and functions. The choice of heir belonged to the Head of State alone, to be confirmed in the Cortes Generales by a two-thirds majority and subsequently passed on through inheritance.
The Organic law as well stipulated the Council of the Realm to be reduced from 17 members to 13 (ten elected by the Cortes, seven appointed by the Chief of State), and of the Cortes from 611 deputies to 403 (108 directly elected, 25 appointed by the Head of State, with the rest elected by corporate organs). It also confirmed freedom of religion for Spanish Jews and Protestants. It was reportedly approved by 98.1% of voters.
The new constitution took effect as of January 1, 1967, and remained in place until the present Spanish Constitution of 1978 was enacted by a referendum the same year, following Franco's death and the resumption of parliamentary government through a fully and directly elected assembly.