In the pre-recorded seven-minute video, the father-and- son, speak emotively about the death of endangered species, including elephants, which the Prince of Wales said are being slaughtered at a rate of 100 per day.
Their first-ever joint video message was released ahead of a major international conference in London on Thursday to help eradicate illegal wildlife trade and better protect the world's most iconic species from the threat of extinction.
In the message, recorded at Clarence House in November, Charles says: "More than 30,000 elephants were killed last year, amounting to nearly 100 deaths per day.
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"As recently as 100 years ago, there were as many as 100,000 wild tigers living in Asia. Today, there are believed to be fewer than 3,200 left in the wild," Charles, also president of the wildlife charity WWF-UK, says.
William, the Duke of Cambridge said the birth of his son, Prince George, has fuelled his passion to protect endangered species and their habitats.
"This year, I have become even more devoted to protecting the resources of the Earth for not only my own son but also the other children of his generation to enjoy," he said.
In the video, Prince Charles is heard for the first time speaking in Mandarin before using his Arabic and Spanish. Then William speaks in Swahili and in Vietnamese.
William, who quit the British military last year to head a new global conservation group called United for Wildlife.
Meanwhile, the release of the message of Charles and William came when Britain's 'The Sun' newspaper reported that the Duke of Cambridge was in Spain on a shooting holiday with younger brother Prince Harry, where the pair will hunt wild boar and stag on the private estate of William's Godfather, the Duke of Westminster.