Google will display warnings above the search results for 13,000 terms it believes are associated with more explicit child sexual abuse terms, it announced Monday. Microsoft said it will take similar action on its Bing search engine, and on Yahoo searches powered by Bing.

The two companies are acting at the request of U.K. Prime Minister David Cameron, although a Google spokesman said the changes will affect searches worldwide, not just in the U.K.

The moves were criticized by online rights campaigners, who said the measures were more about preventing damage to the companies' reputations, and would not be very effective in protecting children.

In July, Cameron called on Google and Microsoft, which together cover 95 percent of the search market in the U.K., to block results for certain searches to make sure that no illegal content or pathways to illegal content were returned.

The U.K.'s Child Exploitation & Online Protection Centre (CEOP) provided the search engines with a list of terms it said were unambiguously used by those looking for child abuse images online, Cameron's office said in a news release.

When people search Google with a term linked to child abuse, clear warning messages from Google and child safety organizations are displayed explaining the consequences of their actions and pointing them toward expert help, the company said.

Google also introduced changes to prevent content such as images and peer-to-peer links to child abuse material from appearing for more than 100,000 unique searches associated with child sexual abuse terms, it said. It also developed and agreed to share a new technology that allows copies of videos of child abuse to be identified and removed, it said

Microsoft's Bing search engine.

The video identification system is similar to Microsoft's PhotoDNA, which associates unique identifiers with still pictures, allowing child abuse images to be identified and removed from online services.

Microsoft will help design a new U.K. national image database that will use PhotoDNA identifiers to track and take down images, it said in a news release. Microsoft too will display warning messages when certain search terms are used on its Bing search engine, or for Yahoo searches powered by Bing, it said.

Google and Microsoft will collaborate with CEOP, the Internet Watch Foundation (IWF) and the U.K. government.

The companies, CEOP and IWF will also take part in a joint work program to help prevent sharing of child sexual abuse content via torrents, they said. The goal is to establish a new reporting process to remove peer-to-peer links to child sexual abuse.

"These measures will have a significant impact on our ability to tackle child abuse imagery online," said IWF in a statement. CEOP and IWF declined to comment on the matter until after their meeting with Cameron on Monday.

However, doubts were raised about effectiveness of the measures taken by the search engines on request of the government. The CEOP, for instance, concluded in a July 2012 study that many U.K. sexual offenders use the Web anonymously.

The director of the U.K.'s Open Rights Group, Jim Killock, pointed out in a blog post that many child protection experts are questioning the measure's usefulness. "We don't know, as we've seen no evidence, whether Google and Microsoft really will be able to make a difference by limiting search, or whether the actions are cosmetic," he said, adding that Google and Microsoft have always removed search results such as URLs or images that are reported to them.

Monday's announcement only proves Internet companies are susceptible to pressure and will take action when threatened, Killock wrote. "It doesn't matter if the companies are at fault, or can only make a limited difference, but when accused of aiding paedophilia, they are certain to take action to limit the reputational damage," he said.

"If David Cameron and his advisor Claire Perry are hyping a policy they know is of marginal importance, many people will be forced to conclude that the announcement is a cynical manipulation of parents' fears in order to appear to be taking action," he said.

The efforts are probably not going to be very effective, agreed Joe McNamee, director of civil rights group European Digital Rights, in an email. "The companies are taking arbitrary but press-friendly measures as a way of seeking refuge from bad publicity," he said.