Despite drastic penalties and continued prohibition of opium until 1860, opium smuggling rose steadily from 200 chests per year under the Yongzheng Emperor to 1,000 under the Qianlong Emperor, 4,000 under the Jiaqing Emperor, and 30,000 under the Daoguang Emperor. The illegal sale of opium became one of the world's most valuable single commodity trades and has been called "the most long continued and systematic international crime of modern times". Opium smuggling provided 15 to 20 percent of the British Empire's revenue and simultaneously caused scarcity of silver in China.