Microsoft's Bing search engine is the first in the UK to add pop-up warnings for those searching for images of child abuse.
The pop-up notification will inform the user that the content is illegal and give a link to the Stop it Now counselling service. The warning will be triggered by a 'blacklist' of words compiled by the Child Exploitation and Online Protection Centre (Ceop).
Microsoft said the pop-up aims "to stop those who may be drifting towards trying to find illegal child abuse content on the web via search engines"
"This is in addition to Microsoft's existing and longstanding policy of removing any verified links to illegal content of this sort from Bing as quickly as possible." a spokesman told the BBC.
Yahoo, which uses Bing technology, is considering the same change. The UK's most popular search engine, Google, is not planning to implement the same pop-up system but will continue to report material and assist experts.
The move comes after Prime Minister David Cameron said internet firms need to do more to block online access to such content and other pornography. However, Microsoft's changes to Bing don't go quite go as far as to not give any search results at all.
Andy Baker, chief executive of Ceop, said: "This is a positive step in the right direction to deterring potential offenders from accessing indecent images of children on the internet. But it is a small, initial part of the solution to prevent child sexual abuse, protect children and pursue offenders."
"While the Bing project isn't the whole solution, I hope it goes some way to making those who are curious about searching for indecent images think again." he added.