More than 25 business websites around the world have been hacked to show images of child sex abuse.
The Internet Watch Foundation (IWF) has received 227 reports over the past six weeks.
The IWF gave the BBC one example of a retailer’s computer servers attacked by creating an “orphan folder” where hundreds of images were then uploaded. This created a new section on its website – which could go unnoticed until visitors complained.
The IWF said the images appearing on sites were the “worst of the worst" sexual abuse, including very young children.
Legal pornographic sites were also attacked and links to “adult images” were hacked so that they redirected the user to images of child abuse.
The IWF said that in some cases the hacked sites were also infected with malware.
This news comes shortly after prime minister David Cameron announced that the UK would block online pornography by default to all new internet users. Users would have to ask for filters to be turned off if they want access.
The anti-porn measures include steps to stop children accidentally finding explicit, but legal, pornographic images in public places.
The move comes after access to illegal child pornography was linked to two men convicted of high-profile child murders.
Bing's pop-up warnings will be triggered only by searches conducted in the UK using terms on a list compiled by the Child Exploitation and Online Protection Centre (Ceop).
The notification will tell them the content is illegal and provide details of a counseling service.