In a pre-recorded video message played yesterday to delegates at the opening ceremony of the summit held in Thailand, William said entire species could disappear from the wild if nothing was done.
The Duke said: "As we enter 2013, the world's natural resources are under threat as never before. We know from the data and analysis presented to this meeting that the illegal killing of the African elephant and rhino, and the related illegal trade in their ivory and horn, has reached shocking levels in the past few years."
He added: "We must do more to combat this serious crime if we are to reverse the current alarming trends. If not, we could soon see some populations of these creatures, or even an entire species, disappear from the wild.
"We simply must not let this catastrophe unfold. Our children should have the same opportunity that we have to experience wildlife in its many beautiful and varied forms."
His message was broadcast at the Bangkok conference of Cites (Convention on International Trade and Endangered Species of Wild Fauna and Flora).
About 2000 delegates representing 177 governments, indigenous peoples, non-governmental organisations and businesses are due to attend the 12-day conference to decide measures to combat overfishing, illegal logging and wildlife crime.
William is royal patron of the wildlife conservation charity Tusk Trust, which works in Africa on projects to preserve endangered animals on the continent.
At the conference, Thai Prime Minister Yingluck Shinawatra said Thailand would end its domestic ivory trade.
She promised legislation that could help the country avoid trade sanctions after criticism by environmental groups.