WIESBADEN, Germany — Five of Germany's eight major Internet service providers — Deutsche Telekom's T-Online, Vodafone's Arcor, Kabel Deutschland, Telefonica's O2 and Alice's Hansenet — signed a legally binding agreement with the government and the Federal Crime Office on Friday, agreeing to install software to block consumer access to child pornography sites. The five companies together cover around 75 percent of the German market.

Software blocks installed by the ISPs will redirect consumers attempting to click on blacklisted websites to a red stop sign. The Federal Crime Office has compiled a blacklist of 1,000 sites, which is updated daily.

Under the agreement, the ISPs have six months to reprogram thousands of servers and install the page blockers. The government expects that, once these are in place, as many as 450,000 attempts to access child pornography sites will be blocked daily.

German Family Minister Ursula von der Leyen proposed the law, modeled on similar action taken years ago in Scandinavia, Britain and Italy.

"If these countries can overcome any legal and technical issues and successfully fight child porn online, we can do the same in Germany," von der Leyen said.

Other Internet providers who service the remaining 25 percent of the German market have not joined in the agreement, saying they do not want to risk breaching the telecommunications secrecy law by blocking child pornography sites. They also argue that they only provide the technical capacity to distribute information and are not responsible for content that is passed through their networks.

Limiting access to information is a sensitive subject in Germany because of the country's Nazi history and East German Communist rule, but von der Leyen said protecting children was the priority.

"The vulnerability and dignity of children is more important than mass communication," she said.

The German cabinet is expected to announce changes to the telecommunications law by summer that would force the remaining Internet providers to block child porn sites.

There's no reliable information on the extent of child pornography, but the German government says access to video and other images of child pornography on the Internet more than doubled from 2006 to 2007.

Earlier this week, German police announced they had smashed a global ring of some 9,000 suspected pedophiles in 92 countries. Pornographic images of children were transmitted from more than 1,000 connections in Germany to 8,000 IP addresses in countries including the United States, Canada, New Zealand, Austria and Switzerland. The footage included "images of the most serious sexual abuse, even of toddlers,” police said in a statement.

Critics have said that the new law is practically useless, because companies abroad can easily bypass the software block. According to German computer magazine CT, a 27-second online video is already circulating showing how the block can be circumvented. Only the "naive Internet user" might possibly be stopped from accessing prohibited material, the magazine wrote.

"Most child pornography material is available through private forums, like exchange sites, newsgroups, chat rooms, free areas of Usenet and email distribution lists," said Udo Vetter, a lawyer who often represents people charged with possession of child pornography, In an interview with CT.

Vetter said "many simply receive the videos on DVD via the mail," adding that he doubted a major child pornography industry even exists. Vetter estimated that 98 percent of such images have been around for years and that the quality of most new material that surfaces seems to have been made by private individuals in a domestic setting.