A strong opponent of slavery, he became a founder member of the Committee for the Suppression of the Slave Trade. He collected much information about the trade and conditions on slave ships, which was published in a pamphlet and used by William Wilberforce in his parliamentary campaign for abolition. In 1807 an Act was passed prohibiting British participation in the slave trade. In 1823 Clarkson became a leading member of the Anti-Slavery Society, which saw its efforts rewarded with the 1833 Act abolishing slavery in the British empire.