A US Department of Justice-ordered study into pornography on the internet has found that just over 1 per cent of web pages contain sexually explicit material, according to details revealed in a court hearing.

While representing only a small proportion of all material on the internet, the findings still showed that hundreds of millions of freely available web pages contain adult material, experts said. That could add to pressure in the US for a fresh legal crackdown on online pornography, a move that has been blocked for years by free speech advocates.

“Measuring anything in percentages is a strange way to go about it because there is not limited real estate on the web,” said Jonathan Zittrain, professor of internet governance and regulation at Oxford University. “It’s not like saying how much of a town is given over to its red-light district.”

Yahoo!, for instance, said a year ago that its search engine had indexed 24bn web pages and Google has consistently claimed to reach far deeper into the internet.

The justice department study was ordered as part of a long-running legal dispute over the Child Online ­Protection Act (Copa). Passed in 1998 to impose penalties on websites that publish pornographic material, enforcement of the act has been held up by an injunction from the American Civil Liberties Union.

The study by Philip Stark, a professor of statistics at the University of California, Berkeley, was carried out to discover how much pornography is available to the general internet user and how effective the filters of search engines are at identifying and removing it.

It became the focus of heated debate this year when Google, fearing the precedent it would set, refused to hand over any information from its search engine to Mr Stark. The company was eventually ordered by a court to comply, although it was allowed to withhold some of the detail that Mr Stark had requested.

According to Mr Stark’s study, 1.1 per cent of web pages contain explicit material and 6 per cent of searches return results that link to pornography.

The findings, first reported in the San Jose Mercury News, were disclosed in Federal court in Philadelphia last week during the latest hearing into the ACLU’s injunction.

Although a large amount of pornography was still available on the internet, threats of legal action in the US had at least succeeded in making much of it unavailable to children and other casual internet surfers, said Mr Zittrain.

In anticipation that laws such as Copa would eventually be enforced, he said: “We have seen most commercial pornography sites going behind credit card ­barriers.”

That runs counter to the broader trend on the internet, which has seen most “content” producers opt for free, advertising-supported sites

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