One of David Cameron's most high-profile celebrity advisers has branded the Prime Minister's plan to restrict online porn as "an absolutely ridiculous idea that won't work."

Jimmy Wales, co-founder of Wikipedia, was asked about the plans for web users to opt-in to receive adult content, and otherwise be blocked.

Warning that access to online pornography is "corroding childhood" the Prime Minister said internet service providers will ask users who open new contract whether they want to activate filters which restrict adult material.

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Without an answer either way, the filters will be activated by default.

Mr Wales told Channel 4 News: "When Cameron uses the example of paedophiles who are addicted to internet porn – all that these plans would do is require them to opt in. It's an absolutely ridiculous idea that won't work."

He said that - given more money - police could use existing legislation to battle the problem.

Turning to the revelations about the US National Security Agency's trawl of web use data, Wales decried that "billions had been wasted shopping on ordinary people's data in a fruitless search for terrorists".

He said: "We should be devoting a significant proportion of that to dealing with the real criminal issues online, stealing credit card numbers, hacking into sites ... that is going to take an investment in real, solid police work."