British internet providers will begin blocking access to online pornography unless customers specifically opt-in to surf sexually explicit material, Prime Minister David Cameron announced Monday.

Cameron relayed the move as part of a series measures to stop extreme sexual images that he says are "corroding childhood."

"In the darkest corners of the internet, there are things going on that are a direct danger to our children, and that must be stamped out," he said in a speech.

Cameron said that "family-friendly" filters would become the default setting for new customers by the end of the year, and only account holders who meet age-verification protocols would be able to change them.

Britain's largest internet service providers, who have agreed voluntarily to the government plan, are expected to contact current customers by year's end and give the option to either filter out explicit material or have unfettered access. Customers who don't choose will have filters applied.

Cameron's speech said that a "free and open internet is vital," but that "in no other market — and with no other industry — do we have such an extraordinarily light touch when it comes to protecting our children."

"Children can’t go into the shops or the cinema and buy things meant for adults or have adult experiences — we rightly regulate to protect them." he said. "But when it comes to the internet, in the balance between freedom and responsibility, we have neglected our responsibility to our children.

"My argument is that the internet is not a side-line to 'real life' or an escape from 'real life;' it is real life."

'Default censorship'

The prime minister also announced a plan to criminalize possession of violent pornography containing simulated rape scenes, and said Google and other search engines would be asked to block searches based on certain "horrific" phrases.

Anti-pornography activists welcomed the announcement, but Padraig Reidy, with the free speech group Index on Censorship, said it amounted to "a kind of default censorship."

"If a filter is set up as a default then it can really restrict what people can see legitimately," he told BBC radio. "Sites about sexual health, about sexuality and so on, will get caught up in the same filters as pornography. It will really restrict people's experience on the web, including children's."

Cameron also unveiled new initiatives to combat child pornography, calling on search engines like Yahoo and Google, who he said must take greater responsibility to block explicit images of kids. He said they have a "moral duty" to block such content, and dismissed any objections based on freedom of speech.

The prime minister recently met with the parents of two murdered British schoolgirls who reportedly said that child pornography fueled the violent acts of their childrens' killers.

"They want to feel that everyone involved is doing everything they can to play their full part in helping to rid the internet of child abuse images," he said.

Columnist Nick Cohen, of the Spectator magazine, said recent disclosures about government snooping should make people wary of giving companies and authorities personal information — such as a desire to view pornography.

"The expansion of legislation prohibiting pornographic images may sound equally reasonable until you remember it gives more powers to police and prosecutors," he wrote. "The record shows they cannot be trusted to use them justly."

The U.K.'s biggest internet providers, including BT, Sky and Talk Talk have reportedly agreed to Cameron's plan, meaning it will cover some 95 per cent of homes.

With files from the Associated Press