The Duke of Cambridge has called people involved in the illegal trade of rhino horn "extremely ignorant, selfish and utterly wrong".
Black rhinos are critically endangered in the wild because of the soaring price of rhino horn, which is worth more than gold due to demand in some countries.
Rhino horn is perceived to possess medicinal properties and in some Asian countries it is believed that it holds a cure for cancer and acts as an aphrodisiac.
Asked by BBC Breakfast what he thought of people who knew of the rhino's plight but continued to poach or buy their horns, William replied: "I think they are extremely ignorant. I think they are selfish. I think they are wrong, totally and utterly wrong.
Angry: The Duke of Cambridge condemned the illegal trade of rhino horn
"It makes me very angry, it's a waste."
Poaching has had a dramatic impact on all rhino populations in the wild and it is believed they could soon be extinct if they continue to be killed.
The Duke said: "If we don't do something about them it is going to be a tragic loss for everyone."
Asked about those who seek rhino horn, William said: "My message to them is simply 'Stop'.
"It's a message about educating people and understanding that when you buy that rhino horn, or when you buy that ivory, you are taking this from an animal that has been slaughtered for this decorative ornament you have on your mantelpiece and you have at home - is that really what you desire and what you feel is right in the world?"