LIMA, Peru—The illegal clearing of land for commercial agriculture is the cause of almost half of tropical deforestation globally, with much of those commodities destined for international markets, according to a study released Wednesday.

The study, which was prepared by Washington-based nongovernmental organization Forest Trends and was funded by the United Kingdom's Department for International Development, looked at the role of agriculture in deforestation of tropical forests from Peru and Brazil to Malaysia and Congo from...