A European Union proposal to stop people from accessing bomb-making instructions online is fraught with technical difficulties, if not downright unworkable, Internet practitioners say.
EU Justice and Security Commissioner Franco Frattini says he is working on plans that would block Web searches for bomb recipes and oblige Internet Service Providers (ISPs) to prevent access to sites containing them.
At present, crude bomb-making guides are readily available.
"I do intend to carry out a clear exploring exercise with the private sector ... on how it is possible to use technology to prevent people from using or searching dangerous words like bomb, kill, genocide or terrorism," Frattini told Reuters in an interview earlier this month.
Frattini is expected to present his proposals in the next few weeks as part of a package of anti-terrorism measures. They are likely to spur fierce debate over feasibility and cost.
Some in the Internet industry are adamant that such ideas are doomed to fail.
"They will not work because it doesn't make sense to block something," Harald Summa, chief executive of the association of German Internet enterprises, Eco, told Reuters.
"If you block it, probably the same content is available on a different site within a minute, so this doesn't help."
Other Web industry sources did not dismiss the EU idea out of hand but noted that the precedents are difficult.
"It's not easy but there are situations in which we do it already," said a London-based spokesman for Google .
"For example, in France and Germany it's illegal to show certain Nazi-related sites, so they are removed from google.fr or google.de search results, as appropriate. It doesn't mean that if you search for the word 'Nazi' on google.fr, you get nothing. It just means that results that we have identified ... as being illegal in that jurisdiction do not show up."
The spokesman said, however, that blocking searches was a "serious technological proposition" and noted that there are ways to get around it.
In the Nazi example, French-speakers can get unfiltered search results by going to the French-language version of Google Canada. And politically sensitive content which is filtered out on Google China can be found by going to google.com instead and searching in Chinese, he said.
In practice, Web experts said, EU attempts to combat objectionable content on the Web would depend whether it was hosted inside or outside the bloc, and whether it was made illegal or merely declared undesirable.
There is no problem getting EU-based providers to close down sites that are clearly illegal, such as those dealing in child pornography, although ISPs say it is not their job to police billions of Web pages and so the onus is on the authorities to bring criminal content to their attention.
Things get harder if the illegal content is hosted outside the 27-nation EU.
One option is to compile blacklists of illicit foreign sites and get ISPs to bar access to them for their customers. For example, Britain's Internet Watch Foundation has a list of up to 1,200 foreign child sex abuse Web addresses which it circulates to providers, a spokeswoman said.
But Richard Nash, head of the European Internet Services Providers' Association (EuroISPA), said such an approach had severe limitations.
"Putting such a block in place may sound easy on the face of it, but that assumes these Web sites remain in the same place, which they don't. It moves around, that's how they avoid being caught," he said in a telephone interview.