Prince William went on a deer and wild boar hunting trip in Spain a day before he launched a high-profile appeal to stop the illegal hunting of wildlife, it has emerged.

There is no suggestion the hunting trip was in any way illegal, but critics have suggested that the timing of the holiday was poor, coming before an appeal close to the prince's heart.

In a message broadcast on Sunday, the Duke of Cambridge and his father, Prince Charles, called on people to help save endangered animals such as rhinos, elephants and tigers.

A royal spokesman said the duke, who quit the RAF last year to lead a new global conservation group, United for Wildlife, was a "passionate advocate" on the subject.

The BBC royal correspondent Nicholas Witchell questioned the timing of the trip, which was also attended by Prince Harry, the Sun reported. "Such a trip is a world away from shooting endangered species for profit, but some may feel that to go hunting himself just as he launches a high-profile campaign is, at the very least, ill-timed," Witchell said.

A royal spokesman said: "The Duke of Cambridge has for many years been a passionate advocate for endangered wildlife and has campaigned tirelessly to help stop the illegal poaching of rhino horn and elephant tusk. His track record in this area speaks for itself."

In the broadcast, William and Charles make appeals in languages including Mandarin, Arabic and Swahili to stop the illegal wildlife trade.

Prince Charles, president of the wildlife charity WWF UK, starts the message with a warning that the trade has reached "unprecedented levels of killing and related violence" and poses a threat not only to endangered animals but to economic and political stability around the globe.

He says: "More than 30,000 elephants were killed last year, amounting to nearly 100 deaths per day. In the past 10 years, 62% of African forest elephants have been lost. If this rate continues, the forest elephant will be extinct within 10 years. A rhinoceros is killed every 11 hours. 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."

The duke, who is royal patron of the wildlife conservation charity Tusk Trust, says he wants future generations, including his son, George, to be able to appreciate protected wildlife. "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 says.

On Wednesday the duke will attend a United for Wildlife symposium at the Zoological Society of London, followed by an evening reception at the Natural History Museum to mark the start of a high-profile wildlife conference hosted by the British government.

The conference, held at Lancaster House in London, will host policymakers and campaigners from 50 countries, and there will be speeches from Charles and foreign secretary, William Hague.

The 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, according to the government.