Leonhard, the chief executive of the Futures Agency, opened the day's final panel session, which also included figures from Incisive Media, Mindshare UK, Osborne Clarke and Exponential.
He said the world was "at war" over data through recent privacy lawsuits against Facebook and the revelations earlier in the year of the US government's Prism surveillance programme.
Advertisers should create a "new social contract" by building trust with consumers over how they use data, Leonard said.
He referenced the various "digital bills of rights" created by the White House and the website Mashable, suggesting advertisers need to create similar commitments to use consumer data transparently.
Leonhard said: "I think it's really important that advertisers and brands get behind these digital bills of rights, for their own good. If trust is broken, business breaks as well. You cannot do business in a world with no trust."
Leonhard said advertisers, like governments, had become obsessed with becoming "superpowers" by gathering as much data as possible from consumers. He asked: "Do we want to live in a world where what fuels surveillance also fuels advertising? Is that what we want?"
He continued: "People think nobody gives a shit about this except futurists, but the next story is that if we abuse shared data and people's openness, then those people will disconnect from us.
"They are doing this already; we just haven't noticed it. Think about the argument of Facebook, if things go on as they are in terms of data use, it will be dead in three years.
"I just moved all my data off Gmail. I love Gmail but I don't like its game. Abuse is not a sustainable business model. Obama saying that we have to be either private or secure is ridiculous. I've never heard of such bargain."
Mark Creighton, the chief executive of Mindshare UK, agreed that we are at "a juncture".
Creighton said: "For the first time everyone understands what data means, and we're seeing the people that are receiving these cookies as people. What is the value exchange that we give to consumers? Having to explain cookies to consumers was the accelerator we needed to explain things clearly."