BOGOTÁ, Colombia — The government of Colombia on Thursday night rejected a major tool in the American-backed antidrug campaign — ordering a halt to the aerial spraying of the country’s vast illegal plantings of coca, the crop used to make cocaine, citing concerns that the spray causes cancer.

The decision ends a program that has continued for more than two decades, raising questions about the viability of long-accepted strategies in the war on drugs in the region.

Colombia is one of the closest allies of the United States in Latin America and its most stalwart partner on antidrug policy, but the change of strategy has the potential to add a new element of tension to the relationship.

Just last week, American officials warned that the amount of land used to grow coca in Colombia grew by 39 percent last year as aerial spraying to kill or stunt the crop, already a contentious issue here, declined.