In 1807 the Houses of Parliament in London enacted legislation prohibiting British subjects from participating in the slave trade. Indirectly, this legislation was one of the reasons for the collapse of Oyo. Britain withdrew from the slave trade while it was the major transporter of slaves to the Americas. Furthermore, the French had been knocked out of the trade during the French Revolution beginning in 1789 and by the Napoleonic wars of the first fifteen years of the nineteenth century. Between them, the French and the British had purchased a majority of the slaves sold from the ports of Oyo. The commercial uncertainty that followed the disappearance of the major purchasers of slaves unsettled the economy of Oyo. Ironically, the political troubles in Oyo came to a head after 1817, when the transatlantic market for slaves once again boomed. Rather than supplying slaves from other areas, however, Oyo itself became the source of slaves.

British legislation forbade ships under British registry to engage in the slave trade, but the restriction was applied generally to all flags and was intended to shut down all traffic in slaves coming out of West African ports. Other countries more or less hesitantly followed the British lead. The United States, for example, also prohibited the slave trade in 1807 (Denmark actually was the first country to declare the trade illegal in 1792). Attitudes changed slowly, however, and not all countries cooperated in controlling the activity of their merchant ships. American ships, for instance, were notorious for evading the prohibition and going unpunished under United States law. It should be noted, moreover, that the abolition movement concentrated on the transatlantic trade for more than five decades before eventually becoming a full-fledged attack on slave trading within Africa itself.

The Royal Navy maintained a prevention squadron to blockade the coast, and a permanent station was established at the Spanish colony of Fernando Po, off the Nigerian coast, with responsibility for patrolling the West African coast. For several decades, as much as one-sixth of all British warships were assigned to this mission, and a squadron was maintained at Fernando Po from 1827 until 1844. Slaves rescued at sea were usually taken to Sierra Leone, where they were released. British naval crews were permitted to divide prize money from the sale of captured slave ships. Apprehended slave runners were tried by naval courts and were liable to capital punishment if found guilty.

Still, a lively slave trade to the Americas continued into the 1860s. The demands of Cuba and Brazil were met by a flood of captives taken in wars among the Yoruba and shipped from Lagos, while the Aro continued to supply the delta ports with slave exports through the 1830s. Despite the British blockade, almost 1 million slaves were exported from Nigeria in the nineteenth century. The risk involved in running the British blockade obviously made profits all the greater on delivery.

The campaign to eradicate the slave trade and substitute for it trade in other commodities increasingly resulted in British intervention in the internal affairs of the Nigerian region during the nineteenth century and ultimately led to the decision to assume jurisdiction over the coastal area. Suppression of the slave trade and issues related to slavery remained at the forefront of British dealings with local states and societies for the rest of the nineteenth century and even into the twentieth century.

Lagos, where the British concentrated activities after 1851, had been founded as a colony of Benin in about 1700. A long dynastic struggle, which became entwined with the struggle against the slave trade, resulted in the overthrow of the reigning oba and the renunciation of a treaty with Britain to curtail the slave trade. Britain was determined to halt the traffic in slaves fed by the Yoruba wars, and responded to this frustration by annexing the port of Lagos in 1861. Thereafter, Britain gradually extended its control along the coast. British intervention became more insistent in the 1870s and 1880s as a result of pressure from missionaries and liberated slaves returning from Sierra Leone. There was also the necessity of protecting commerce disrupted by the fighting. The method of dealing with these problems was to dictate treaties that inevitably led to further annexations.

Data as of June 1991