Google was the number one third-party company. The research found that Google, or one of its subsidiaries like the advertising platform DoubleClick, had trackers on 74 per cent of the pornography sites examined. Facebook had trackers on 10 per cent of the sites.

We are told that those who regularly visit porn sites turn on an incognito mode on their browsers. But, the researchers said, the incognito mode only ensures that your browsing history is not stored on your computer.

"In the U.S., many advertising and video hosting platforms forbid 'adult' content. For example, Google's YouTube is the largest video host in the world, but does not allow pornography," the researchers wrote. "However, Google has no policies forbidding websites from using their code hosting (Google APIs) or audience measurement tools (Google Analytics). Thus, Google refuses to host porn, but has no limits on observing the porn consumption of users, often without their knowledge."