Google has begun scouring the email accounts of its users so it can warn the police of child abusers.
The technology giant has developed software that identifies child abuse images and automatically alerts the authorities.
The breakthrough means paedophiles around the world will no longer be able to store and send the illegal images by email unknown to the authorities.
Details of the software emerged after a 41-year-old convicted sex offender was arrested in Texas for alleged possession of child abuse images.
It is the first known case in which Google has alerted police about the activities of paedophiles.
American police disclosed that Google's sophisticated search system had identified suspect material sent in an email from Houston.
Child protection experts were automatically tipped off. They alerted police, who swooped after requesting the user's information from Google.
It is hoped the software will play a significant role in the fight against paedophiles who have used the internet to operate in the shadows.
Google, which has been criticised for not doing enough to tackle paedophilia online, has been developing highly specialised software for several years.
In 2008 it rolled out new technology that helped the authorities trace those who were using its search engine to look for illegal images.
While the company refused to comment on this latest case, the arrest in Texas confirms that the software is now being applied to scan the email service.
Google's Gmail is the world's largest free web-based email service with more than 425 million users worldwide.
It is understood that the software compares images held in users' accounts against a vast database of child abuse images collated by child protection agencies around the world.
Each one of the images is given a unique fingerprint, known as a hash, which is then used to compare with those held in the database.
The system operates automatically. No Google employees can see any of the images being examined.
If a match with an image on the database is found a red flag is raised and one of the child protection agencies such as the UK's internet Watch Foundation or the National Centre for Missing and Exploited Children in the US is alerted.
Trained specialists then decide whether to alert the police.
While the technology will be seen as a huge boost to the fight against child abuse and exploitation, the ability of Google to look into people's personal email accounts has raised issues around personal privacy.