Freedom of speech campaigners and independent internet firms have issued a stark warning that the automatic blocking of pornographic websites is “censorship by default” and constitutes a “blunt tool” for dealing with inappropriate content.

On Tuesday it was revealed that Sky had become the first major internet service provider (ISP) to start automatically blocking pornographic websites by default.

The move, which has been condemned as a danger to freedom of expression by campaigners, was prompted by pressure from Prime Minister David Cameron for ISPs to make online filtering mandatory, saying that it was the best way to protect children online.

According to Sky its customers will see a message reminding them to make a choice about filtering when they visit a page deemed unsuitable for children under the age of 13. At this point they can choice to accept the current setting or turn the filter off to visit the page.

Sky announced the move, which will affect up to five million customers, in a blog post as part of a “family friendly” initiative to increase online safety, but the plans have been criticised by anti-censorship groups, who warned that sites about sexual health and sexuality could inadvertently get caught up in the ban.

“The government has persuaded ISPs to give their customers an ‘unavoidable choice’ about filters but it seems that Sky will activate them automatically if their customers don’t respond. This is censorship by default,” said Pam Cowburn from the Open Rights Group, which campaigns for internet freedom. “We know many websites get mistakenly blocked by filters, including those that offer advice and support to young people. Filters should not be switched on automatically. Customers should be given enough information to make an informed choice about whether they want them.'

Jodie Ginsberg, chief executive of the Index on Censorship, added: "Default filtering is a blunt tool for dealing with the problem of children and young people accessing inappropriate content online.

If customers want to filter, they should be given the option to make that choice, rather than a TV company making decisions on everyone's behalf."

The move by Sky follows Cameron’s call in 2013 for automatic online filtering. Since then all the UK's big four ISPs, who between them have more than 20 million subscribers, have implemented filtering software for parents, but not by default.

Since the voluntary changes the Open Rights Group has recorded example of benign websites being blocked after the government put pressure on ISP to clamp down on extremist websites, while Sky has also faced criticism from smaller independent ISPs.

Adrian Kennard, the owner of independent ISP Andrews and Arnold, said: “This is a very slippery slope and one that we’ve seen getting steeper all the time. Ultimately the government is bullying the large ISPs to do this.”

According to Mr Kennard most customers who come to his firm are attracted its fiercely independent privacy and censorship policy and join as a “statement against censorship of the internet.” Previously the firm said that David Cameron’s plans for automatic porn filters meant that internet users may as well “move to North Korea”.

Domestic security experts have also questioned the announcement by sky. Ed Macnair, chief executive of CensorNet, an online security firm, also said the “blanket ban” by Sky was “too blunt an instrument” and that he “expected a backlash from customers”.

Mr Cameron’s attempts to tackle legal and illegal pornography came after he said the problem was “too big to ignore” amid public concern over paedophiles using the internet share images. However experts from the Child Exploitation and Online Protection centre warned last year that moves to restrict search engine results would be “laughed at” by paedophiles, who downloaded abusive images from peer-to-peer websites, not Google.

In a blog, Sky brand director Lyssa McGowan said: “We're all aware that cyberspace can present security risks, and that the internet isn't universally suitable for children.

”What we're doing now is simply making sure that the automatic position of Sky Broadband Shield is the safest one for all - that's 'on', unless customers choose otherwise.