David Beckham has warned of the "devastating" effects of poaching as he joined the Duke of Cambridge to launch a new campaign using the power of sport to combat illegal trade in wildlife products.
William, president of United for Wildlife, joined forces with the football legend to present the #WhoseSideAreYouOn initiative, which will use social media to raise the profile of the issue.
The Duke, who sat beside Beckham on the stage at the central London event, also gave a short speech in which he spoke of how the "illegal wildlife trade thrives because it is hidden, often invisible, making it easy for criminals".
Beckham, who addressed the specially invited audience of conservationists and campaigners after William prince, said he had been "honoured" to be asked to get involved by the royal.
Speaking of the illegal wildlife trade, the former England captain said: "It's devastating. It really is devastating.
"And we're in a world where our generation, and the younger generation can really, really make a difference and we really need to do it now.
"Are we on side of the criminals? Or are we on the side of the animals? I know what side we're all on and I'm sure you know what side you're all on."
Beckham, who worked closely with William on England's World Cup bid and attended his wedding, heads a team of sporting stars backing the campaign including Wimbledon champion Andy Murray, India's former batsman Rahul Dravid and retired South African rugby captain Francois Pienaar.
Beckham added: "It's something that we all feel very passionate about.
"As lead ambassador I'm very proud to also be part of an amazing team - an amazing array of famous faces that are very important to this organisation and very important to this cause."
William stood with his hands clasped in front of him as a short film was played highlighting the issues facing park rangers who must face the ever present threat of poachers and risk their own lives in doing so.
The Duke described United for Wildlife as a "powerful global alliance of seven of the most influential conservation organisations".
William is also involved through the Royal Foundation of The Duke and Duchess of Cambridge and Prince Harry.
The second in line to the throne went on: "In February we set out our commitments to improve protection and enforcement, reduce demand and to help the business community to tackle international trafficking.
"That was our first step, but we knew we needed to do more to bring this trade into the open. The illegal wildlife trade thrives because it is hidden, often invisible, making it easy for criminals to build and expand.
"We wanted to find a way to show the world what was happening. Join us and help stop illegal poaching."
It is hoped the #WhoseSideAreYouOn debate will gather pace on social networking via Twitter, Facebook, Google Plus and YouTube to raise the profile of the issue.
In the run up to the launch, William and Harry posed side by side for a photo for United for Wildlife's Twitter account, each holding a sign bearing the start date "9th June 2014" and their own signature.
Speaking to reporters afterwards, Beckham said: "I want to make a change, I want to make a difference and If I can do that by just shining a light on something as important as this then it's a good thing."
Asked if he expected to able to influence the parts of the world that drive the illegal wildlife trade, he said: "Obviously I have quite a big following throughout Asia and especially in China and if it can shine a light on something as important as this then it's job done.
"But obviously having the Duke, the future king of England involved in something like this - there's nothing better."
Also speaking at the event at Google Town Hall in Soho was Emmanuel De Merode, the director of Virunga National Park in the Democratic Republic of Congo, who spoke of the dangers faced by rangers from poachers. Two rangers are killed every week and he was shot himself earlier this year.
He said the issue was about more than just conservation. "You have to picture wildlife crime in the context of the overall damage that's being done to people's resources, and in Congo there is perhaps no greater example than that, no more extreme example, because it has provoked a war, and this war has seen the death of over six million innocent people," he said.
"This is a war that's driven by the illegal exploitation of natural resources, of which wildlife crime is a part."
Mr De Merode, who is a Belgian prince, said he would he able to return to Virunga and tell his rangers they were "in incredibly good company now". He was shot in the stomach and legs by unidentified gunmen in April and is thought to have been targeted because of his conservation work.
Last September, William and Beckham teamed up when they recorded two video campaigns against illegal wildlife products for WildAid.
The issue is one that is close to the Duke's heart. In February, he hailed a major international conference in London on the trafficking of endangered wildlife as the '"beginning of the end of this despicable trade''.
World leaders agreed concrete steps to help safeguard endangered animals from poachers - putting their signatures to a declaration of intent.
William attended the event with Harry and their father the Prince of Wales, who gave the keynote speech, as politicians gathered to discuss how best to combat the illegal trade in rhino horn, tiger parts and elephant tusks which fuels criminal activity worth more than an estimated £11.5 billion each year.