Selous Game Reserve, the oldest in Africa, boasted 110,000 elephants 40 years ago. Now it has around 15,000.
The nature conservation group said “industrial scale poaching” – driven by demand for ivory from Asian countries, particularly China – was to blame.
Tanzania’s new president has pledged to tackle the corruption that allows the illegal ivory trade to continue.
The reserve in southern Tanzania is 55,000 sq km (21,000 sq miles) and was named a World Heritage Site by Unesco in 1982.
It was named after the Victorian-era British explorer, hunter and conservationist Frederick Selous, and is famous for its pristine ecosystems and abundance of black rhinos, cheetahs, giraffes, hippos and crocodiles as well as elephants.
But the reserve was placed on a list of “World Heritage Sites in Danger” two years ago when on average six elephants were being killed for their ivory every day.
The report, carried out for WWF by global development advisers Dalberg, said that the reserve had lost 90% of its elephants over the last four decades and if the trend continued, they “could vanish from Selous by early 2022”.
“The elephant population in Selous is now near a historic low, and urgent measures are required to protect the remaining animals and return the population to a stable and sustainable size,” the report said.
It also warned that industrial activities in the reserve, including oil and gas exploration, and mineral extraction, threatened the delicate environment and the wildlife – putting at risk the reserve’s “ability to support local communities, which could lead to increased elephant poaching”.
Fred Kumah, of the WWF, said action needed to be taken both in Tanzania and in Asia.
“The demand from the Asian countries, China being a key one, Thailand and others, are a key and we are working with the Chinese government alongside the other Asian countries that are involved in the illicit trade and transit of the illicit trade,” he told the BBC.
“We have control of our countries, and our governments, our businesses need to stand up for the elephants and for the natural spaces in which they are living in.”