Image caption Wikipedia's Jimmy Wales has been advising the government on technology since March 2012

David Cameron's plan to protect children from obscene material online has been dismissed as "absolutely ridiculous" by one of his advisers.

The prime minister announced last month major web providers had agreed to block internet pornography to new customers unless households opt to access it.

But Wikipedia co-founder Jimmy Wales told Channel 4 News the idea "won't work".

He said police should be given more resources to enforce existing laws.

Mr Wales said: "It's an absolutely ridiculous idea. It won't work. The software you would use to implement this doesn't work.

"Additionally when we use cases of a paedophile who's been addicted to child porn videos online, you realise all that Cameron's rules would require him to do is opt in and say, 'Yes, I would like porn please'."

'Flash and snooping'

Mr Wales, who co-founded online encyclopaedia Wikipedia in 2001, said problems like online child abuse, hacking social media sites and abusive or threatening messages could be tackled without the introduction of new legislation.

Responding to calls for tougher regulation of the internet, he said: "For me, what's interesting about criminal gangs hacking into people's Facebook account is that all of that activity is already illegal.

"I can't think of any new laws that would actually help with that. What would help is actual enforcement.

"My view is that instead of spending literally billions of pounds, billions of dollars, snooping on ordinary people and gathering up all of this data in an apparently fruitless search for terrorists, we should devote a significant proportion of that to dealing with the real criminal issues online - people stealing credit card numbers, hacking into websites and things like that.

"Unfortunately we're not seeing a lot of that. We see a lot of flash and a lot of snooping. But this is, at the end of the day, going to take an investment in real, solid police work."

The PM said last month that an agreement with internet service providers (ISPs) meant that by the end of 2014 every new broadband contract will ask customers to opt in to receive adult content, while existing customers will be contacted and asked to decide whether to use the "family-friendly" filters.

The deal is with the the UK's biggest ISPs - BT, Virgin, Sky and TalkTalk - who account for around 90% of UK internet users.

'History of rules'

Mr Cameron also called on search engines like Google to "blacklist" terms used by paedophiles to hunt for child abuse images.

He said the companies would "rewire their technology" to protect children - but the lack of further detail on the proposals led some experts to suggest they would be unworkable.

Mr Wales said micro-blogging website Twitter should make it easier for users to report abuse, but rejected calls for tighter regulation of the social network after abusive messages, including rape threats and bomb threats, were sent to female users.

He said: "When you think about rules about verbal threats, human society has a long history of rules and laws around this, and those rules and laws are very well thought-out. They deal with complicated cases.

"I do think that Twitter has needed in the past to do more to give people more control of the environment, to allow faster means for people to complain and to have people behaving badly exposed, blocked or arrested as necessary.

"But it is not like we don't have a law against threatening people. We do, and people are quite rightly being called up on this."

Mr Wales was unveiled as an unpaid Whitehall adviser with a brief to advise civil servants on open access to information online in March 2012.