After the abolition of slavery got underway, the Portuguese dragged their heels, so in 1839 the British government declared its right to inspect Portuguese ships for evidence of slave trading with or without Portuguese consent. That stirred the Portuguese to action, and in a subsequent series of agreements made in the 1840s, the British acquired the right to land their ships to land where no Portuguese authorities were present. When the Portuguese refused to renew the agreement in 1853, the British ceased paying tariffs at the ports on either side of the Congo River mouth, claiming that Portugal's claim had expired because they had left the area unoccupied for too long. Portugal reoccupied the ports of Cabinda and Ambriz in 1855, and relations with Great Britain improved after that. The dispute set a precedent, however, that effective occupation was a prerequisite for recognition of colonial claims. The question continued to reappear until 1885 when it was enshrined in the agreements that emanated from the Congress of Berlin.