This may change, as new evidence suggests it is already in use across large parts of the web.

The fingerprinting technique was invented in 2012 by researchers at the University of California, San Diego, and has already been developed into commercial products by companies including AddThis.

In an academic study due to be published soon, researchers from Princeton University and KU Leuven University in Belgium scoured the top 100,000 sites by Alexa ranking and found that 5.5 percent of them already use canvas fingerprinting. Of those, the vast majority (95 per cent) were using code from AddThis - a company which provides social media sharing widgets. On many sites they provide the buttons which allow you to quickly share links via Facebook or Twitter. This gives them a large foothold across many parts of the web - it reaches 1.6 billion different internet users every month, across 14 million domain names.

Sites running the code includes whitehouse.gov, CBS and porn giant YouPorn.com, according to researchers.

The company's approach to canvas fingerprinting draws the phrase "Cwm fjordbank glyphs vext quiz", which uses every letter in the alphabet exactly once, in a small, invisible part of a website - you won't see it, it's done entirely in the background. Other services which used the technology include dating website Plenty of Fish.

AddThis said that if users install an opt-out cookie on their computers then it will not use the data collected for advertisement targeting. Use of the technique has simply been a trial for research and development purposes, it claims.

According to reports in ProPublica, the company did not tell any of the websites that it is used on that it had begun trials using the canvas fingerprinting technology. It now plans to drop the trials because they are "not uniquely identifying enough".

The company did not reply to a request for comment from the Telegraph.

A YouPorn.com spokesperson told ProPublica that the site was "completely unaware that AddThis contained a tracking software that had the potential to jeopardize the privacy of our users." It has removed all AddThis technology from its site.