Web users are being warned about a new tracking system that monitors what they do online.
The warning comes from researchers who discovered the tracking system being used on thousands of websites. The system is said to exploit features of the web's underlying code to generate unique identifiers for each visitor.
Some of the sites said to be using the tracking system were government portals, pornography sites and online shops.
The team to discover the latest tracking system was made up of researchers at Princeton University in the US and the University of Leuven in Belgium. They analysed 5,000 websites and found that many sites had turned away from well-known systems such as small text files called cookies in favour of more subtle methods.
More than 5% of the sites had turned to a technique known as "canvas fingerprinting" to identify visitors.
The researchers said: "The tracking mechanisms we study are advanced in that they are hard to control, hard to detect and resilient to blocking or removing,."
Many web browsing programs now include tools that allow people to stop their activity being tracked across the sites they visit.
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