In the 1850s, the power struggle between the superintendent and the planters coincided with events in international diplomacy to produce major constitutional changes. In the Clayton-Bulwer Treaty of 1850, Britain and the United States agreed to promote the construction of a canal across Central America and to refrain from colonizing any part of Central America. The British government interpreted the colonization clause as applying only to any future occupation. But the United States government claimed that Britain was obliged to evacuate the area, particularly after 1853, when President Franklin Pierce's expansionist administration stressed the Monroe Doctrine. Britain yielded on the Bay Islands and the Mosquito Coast in eastern Nicaragua. But in 1854, Britain produced a formal constitution establishing a legislative for its possession of the settlement in present-day Belize.

The Legislative Assembly of 1854 was to have eighteen elected members, each of whom was to have at least £400 sterling worth of property. The assembly was also to have three official members appointed by the superintendent. The fact that voters had to have property yielding an income of £7 a year or a salary of a £100 a year reinforced the restrictive nature of this legislature. The superintendent could defer or dissolve the assembly at any time, originate legislation, and give or withhold consent to bills. This situation suggested that the legislature was more a chamber of debate than a place where decisions were made. The Colonial Office in London became, therefore, the real political-administrative power in the settlement. This shift in power was reinforced when in 1862, the Settlement of Belize in the Bay of Honduras was declared a British colony called British Honduras, and the crown's representative was elevated to a lieutenant governor, subordinate to the governor of Jamaica.

Data as of January 1992