Proposals for a default block on internet pornography have been criticised as impractical and potentially counterproductive by technology experts and civil liberties campaigners, setting them at loggerheads with groups such as the NSPCC ahead of a potentially stormy government consultation.

David Cameron announced last week that the government would consult on methods to improve online child protection, including a system whereby filters on adult material were set as default. Anyone wanting to access such content would have to "opt in" with their internet service provider.

The idea, advocated by a number of MPs as the best way to safeguard minors, was the chief recommendation of an independent parliamentary inquiry into online child protection, chaired by Tory backbencher Claire Perry and published last month. It has also been vociferously championed by the Daily Mail.

But according to many in the technology community the proposal is unrealistic. They say that a combination of web-aware children and ever-resourceful pornography sites would leave a network-level block struggling to be effective.

Worse, they argue, it could lull parents into a false sense of security and make them believe they need to take no further action. Mumsnet, the popular and powerful online forum, withdrew its support for the idea after an outcry from its tech-savvy members. "If you're a successful porn company with lots of money at your disposal, you're going to constantly change your web address, you're going to be constantly redirecting," said Stuart Miles, founder of technology website Pocket-Lint.com. "It would be impossible to block these things. And that's even after you've taken the decision as to what you deem to be 'acceptable' porn and 'not acceptable' porn."

Supporters of a default block acknowledge that it would not be an instant cure- all and are adamant that the role of parents remains vital. But they insist the threat posed to children by increasingly explicit pornography is such that any measure to combat it should be embraced. "Opt-in", they believe, is the best there is. "It's no panacea at all," said Jon Brown, head of strategy and development at the NSPCC. "Of course there are going to be other ways in which young people are going to be able to access porn should they want it. But it's another inhibitor." Relying on parents to do the work was not always realistic, he added. "While parents of course have a role to play and responsibility … the reality is that some of our more vulnerable children are going to be in homes and in family situations where those controls and that scrutiny isn't in place," he said. A 2008 YouGov survey found that 27% of boys said they accessed pornography every week.

Last year's Bailey report into the sexualisation and commercialisation of children recommended so-called "active choice" as a means of restricting children's access to adult material – a system whereby users are forced to decide whether or not they want controls. The report prompted the "big four" ISPs – Sky, BT, Virgin Media and TalkTalk – to sign a voluntary code of practice according to which they would aim to make "active choice" obligatory for all new customers from October this year. However, as fewer than 5% of customers change their provider during a typical quarter, the measure was not expected to reach a high proportion of users.

A spokesman for the Department for Culture, Media and Sport said "active choice" remained the Government's preferred option, but that it would look at how it could be advanced, for example by examining whether devices such as laptops could come with filters, or whether ISPs could roll out active choice to existing customers. "The experts have told us that prompting parents to make that choice is the best way to get a result because it makes people engage with it and think about it," he said. No date has yet been set for the consultation.

Accusing Cameron of including the "opt-in" proposal in the consultation as a sop to socially conservative voters, Liberal Democrat MP Julian Huppert said: "This is the prime minister trying to say what he thinks will please the Daily Mail readership. There is lots of evidence from the Bailey report and others that active choice – making parents think about it and make a decision – is the best way of actually providing protection for children."

If the government does stick with "active choice", the ISPs will be pleased. They remain implacably opposed to any system whereby they would, as one employee said, in effect "become judge and jury" of the nation's morality.

Their parental controls, they say, offer the best protection for children. TalkTalk's HomeSafe, the only ISP package with a network-level filter, has seen more than 350,000 customers sign on in the past year. In the parliamentary report, such tools are said to "improve content filtering considerably".

Proponents of the "opt in" option are adamant that the ISPs, which make huge profits from selling internet access, should shoulder more responsibility. Perry's report pointed to the precedent set by mobile phone networks, which have already implemented an opt-in filter. While saying that government regulation of the internet should always be done "with the lightest touch", it urged ministers to seek "backstop legal provisions to intervene should industry progress prove inadequate".

The NSPCC's Brown said: "The internet, of course, in terms of its development and history, has been very libertarian and unregulated, and our view is that probably, now, cautiously and sensibly, we ought to be looking at what appropriate controls should be put in place to ensure that parents are supported in helping to protect children from harmful content."

But many see this as a dangerous path to tread. They object to the idea of adults being put on what they say would in effect become a "porn users' register" and argue that no higher power should have the right to determine, in effect, what legal material they should and should not consume.

Jules Hillier, deputy chief executive of sexual health advisory service Brook, whose website was temporarily blocked last year in the mobile phone networks' filter, said the unexpected restriction had made her worry about the ethics, not to mention the efficacy, of such a block. "It did make me think quite a lot about who is it who has the policy responsibility to say 'yes, that site is OK' and 'that site's not'," said Hillier. "Who is it? Because that's not what telephone companies or internet service providers are for."

Jim Killock, of the civil liberties organisation Open Rights Group, said that a network-level block could lead to a phenomenon known as the Streisand effect, whereby trying to suppress something online actually leads to greater publicity.