Environment officials and campaigners have called for a global moratorium on deep-sea mining and on issuing new exploration contracts unless marine ecosystems can be effectively protected.

Thousands of conservationists, scientists, and diplomats voted in favour of the moratorium late Wednesday at the International Union for Conservation of Nature (IUCN) conference in Marseille, France this week, which aims to lay the groundwork for a key global United Nations biodiversity summit in Kunming, China, in October.

Deep-sea mining uses heavy machinery to...