Many abolitionists were infuriated by the Zong massacre, whose details became known during litigation in 1783, when the syndicate owning the ship filed for insurance claims to cover 132–142 slaves who had been killed.

To expand their influence, in 1787 they formed a non-denominational group, the Society for the Abolition of the Slave Trade, which included Anglicans of the established church (non-Anglicans were excluded from Parliament).

In 1788, Sir William Dolben led a group of his fellow Members of Parliament to the River Thames to board and examine a ship being fitted for a slaving voyage.

On 9 May 1788, Pitt introduced a motion to the House of Commons which asked whether parliament should delay its consideration of the slave trade until its next session.

[5] He argued that the great number of anti-slavery petitions that had been presented to the House on this topic meant that a proper consideration of the issue could not occur with so little time left in the current parliamentary session.

[6] Representatives from Liverpool, a city whose merchants controlled much of the British slave trade and whose economy was deeply tied to it, welcomed Pitt's motion.

[8] He said that immediate measures should be introduced to restrict the number of Africans that slave ship captains could take on board, as a means to reduce losses.

But, some abolitionists, such as William Wilberforce, feared that the act would establish the idea that the slave trade was not fundamentally unjust, but merely an activity that needed further regulation.

[19] More recent research suggests that early work was plagued with measurement error by bad record-keeping, and the act reduced crowding-related mortality.