Britain's Prince Charles and Prince William made a father-and-son appeal on Sunday for an end to the illegal wildlife trade, ahead of a major international conference in London.

Charles, the heir to Queen Elizabeth II, and his eldest child William released a video message warning of the "grave threat" to some of the world's most treasured species.

The royals are attending a series of conservation events this week, including a London Conference on the Illegal Wildlife Trade hosted by the British government on Thursday.

"We have come together, as father and son, to lend our voices to the growing global effort to combat the illegal wildlife trade," the 65-year-old Charles says in the recording, in which he and William sit alongside each other, both wearing suits.

William, 31, says in the nine-minute video: "Despite the terrible crisis that we now face, we both continue to be optimistic that the tide can be reversed."

The recording ends with two royals saying the phrase "Let's unite for wildlife!" in Arabic, Vietnamese, Swahili, Spanish and Mandarin, in what palace officials said was a bid to reach countries most affected by the illegal wildlife trade.

- Royal conservationists -

Charles has long been a champion of environmental issues, and his eldest son is following in his footsteps.

William -- who became a father in July when his wife Kate gave birth to Prince George -- quit the British military last year to head up a new global conservation group called United for Wildlife.

On Wednesday William will attend a United for Wildlife symposium at the Zoological Society of London, and then that evening he will be at a reception at the Natural History Museum to mark the start of the British-government-hosted wildlife conference.

On Thursday Charles and William will both attend the full conference at Lancaster House, a Foreign Office building in London. Charles and Foreign Secretary William Hague are both due to give speeches.

Around 50 countries have been invited to the conference.

The British government said the conference would give "leaders from across the world an opportunity to discuss the issue and agree a more coordinated global response".

In their video recording, the princes discuss the "unprecedented" level of killing of endangered animals such as elephants, which they say perish at a rate of 100 a day.

Charles says the trade "now poses a grave threat not only to the survival of some of the world's most treasured species, but also to economic and political stability in many areas around the world."

William adds: "We have to be the generation that stopped the illegal wildlife trade, and secured the future of these magnificent animals, and their habitats, for if we fail, it will be too late."

The video was recorded in November at Clarence House, which is Charles's official residence in London, royal officials said.

The London conference aims to tackle the wildlife trade by strengthening law enforcement, reducing demand for illegal products and supporting sustainable livelihoods for communities in affected areas, the government said.