WPP chief says internet companies have to take responsibility for the editorial content appearing on their sites

The founder of the world's biggest marketing services company, Sir Martin Sorrell, has said he believes revelations about the National Security Agency's Prism internet surveillance programme are a "game changer" that will spark a fundamental rethink of web privacy by web users.

The WPP chief executive said that Prism, which allows the NSA to collect material including search history, the content of emails, file transfers and live chats, according to documents obtained by the Guardian from whistleblower Edward Snowden, is so important that even young people who often have a cavalier attitude to what they reveal online are likely to be concerned about privacy.

"I think Prism and what's happening in the US will have a very significant impact, I think it is game changing," he said, speaking to the Guardian at the Cannes Lions International Festival of Creativity on Wednesday. "I think the privacy issue is going to be raised to a new level by this. It will alter people's views on privacy, even younger people."

He added: "I think even amongst under 35s, people will become very concerned about privacy. It is going to get aired I think quite extensively publicly, I think it is a matter of great public interest."

Wikipedia founder Jimmy Wales, also speaking to the Guardian in a video interview at the Cannes Lions festival on Wednesday, said he thought most people would find the Prism revelations "pretty astonishing".

Wales cautioned that it was still not 100% clear what was going on and it may be many years before all the relevant information is declassified.

But he said it was going to be a serious issue for US president Barack Obama. "Simply because he did make a lot of really positive noises about not doing this kind of thing, or cutting back on it, or being more transparent about it," Wales added. "For this to all come out – it doesn't feel right for the Obama base. I think that's going to be potentially something he has to deal with."

"What I would forecast in the long run is that more and more and more services online are going to go to encryption. Not just to make sure the government is not snooping on people, but just for basic security."

Sorrell said 25% of WPP's £10bn-plus in annual revenues comes from what he calls "data investment management" for advertising and marketing clients.

However, he admitted even he was stunned when he learned the extent of what the US government could do with Prism.

"The fact that the government has access to this data on an organised basis came as a surprise to me," said Sorrell. "And I would pride myself as being perhaps one of those people that knew more about those things than the average."

He said: "I can't imagine what people of my age and much younger think about it and I think it will alter their views. I think it is very significant."

Vivienne Westwood, the fashion designer who appeared at the Cannes festival on Tuesday, hailed Snowden as a hero.

"I'm a huge supporter of Bradley Manning ... but what Snowden did is even more important as we are all directly involved," Westwood said, speaking at a Cannes session as a guest of agency Sapient Nitro to promote her brand of storytelling and campaigning. "People feel huge sympathy for what's happening in Afghanistan – just shooting people – but everybody is directly affected [by the NSA leaks].

"I think it's wonderful what he did, really brave. The really dangerous thing is that people are going to self-censor after this. It's a real problem for free speech."

Sorrell said that he spoke to a number of senior executives from Facebook, Twitter and Google at a private WPP event in Cannes on Tuesday, but they were reluctant to talk about the issue.

"I asked the question about Prism. It is not something people feel comfortable, even in a semi-private or private environment, talking about," he said.

Sorrell has previously said he believes companies such as Twitter, Google and Facebook are "media companies masquerading as technology companies".

He returned to that theme on Wednesday, recalling a debate he hosted at Cannes a few years ago with Google, Yahoo, Facebook and AOL.

"They all answered [that they were a] technology company," he said. "Well they are hiding behind it … If you have responsibility for the pipes, you can't ignore the responsibility for the editorial content. It is no good saying I am just an engineer tightening the nuts on the pipe, making sure it works. You have to be responsible for the editorial content too, you can't abrogate responsibility for it. It is the same thing. They are media owners."

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