Google and Microsoft are to block internet searches for child abuse images.

New software is to be introduced that will automatically block 100,000 "unambiguous" search terms which lead to illegal content, Google chief executive Eric Schmidt said.

The restrictions will be launched in the UK first, before being expanded to other English-speaking countries and 158 other languages in the next six months.

A further 13,000 search terms linked with child sex abuse will flash up with warnings from Google and charities warning the user that the content could be illegal and pointing them towards help.

There has been mounting pressure on internet companies to take action against searching for illegal content.

Mr Schmidt said Google has been working with Microsoft, which owns the Bing search engine, and law enforcement agencies since the summer.

He said: "We've fine tuned Google Search to prevent links to child sexual abuse material from appearing in our results.

"While no algorithm (instructions for software) is perfect - and Google cannot prevent paedophiles adding new images to the web - these changes have cleared up the results for over 100,000 queries that might be related to the sexual abuse of kids."

Google's new technology will also be able to remove thousands of copies of an illegal video in one hit.

When a child abuse video is discovered, the software can attach a unique code to it which can remove all copies from the web.

The system is also designed to identify new code words or terms paedophiles start to use and can block search results for these too.

The measures were revealed by Google and Microsoft ahead of a cyber summit on online child abuse at Downing Street today. Director Pat McKenna has welcomed the decision by the two companies, but said there was much more work to be done.

He said the new development will help unintended browsers from accessing the content, but said those who wish to access the images would not necessarily use Google or Bing to do so.

Mr McKenna called on the Government to do more to identify abuse websites and block them.