The site earmarked for development sits across both green and grey belt land east of the existing Port of Tilbury and Tilbury2, on the north bank of the Thames estuary.
Robins, the charity’s programmes manager, said building on the land would undermine efforts to reverse declining invertebrate numbers.
“We urgently need to see a coordinated masterplan to secure the future of Thurrock’s wildlife,” he added.
But Stuart Wallace, chief executive of Forth Ports, said new habitats created as part of the expansion would “protect and enhance” the area’s nature.
Council meeting papers showed Natural England and the Environment Agency had been consulted on the plan since 2022.
Papers said the scheme’s effects were “fully mitigated and compensated”, with a 69-acre (28ha) “compensation site” planned at Mucking Marshes landfill.