Delegates at an international wildlife meeting in South Africa heard that this came despite growing outrage and calls to dismantle trafficking networks which often collude with government officials.

A document released by conference organisers on Saturday said the illegal ivory trade "has remained fairly constant at unacceptably high levels" since 2010.

Tom Milliken, a report co-author, estimates that there are about 50 ivory seizures of more than half a tonne, and sometimes as many as four tonnes, every year.

Mr Milliken said such big shipments are a sign that organised crime groups are involved in poaching.

Saturday marks the start of the meeting of member nations of the Convention on International Trade in Endangered Species of Wild Fauna and Flora (Cites).