Bolivia is set to withdraw from an international narcotics convention in protest at its classification of coca leaves as an illegal drug. President Evo Morales, who is also the leader of one of the country's main coca producers' unions, has asked Congress to pass a law that would take Bolivia out of the 1961 United Nations Single Convention on Narcotic Drugs. The government says that the convention contravenes the Bolivian constitution, which states that the country is obliged to preserve and protect the chewing of coca leaves as a cultural heritage and ancestral practice.