Facebook has been accused of blackmail by a prominent privacy advocate after the social network sent a letter to the Belgian Privacy Commission (BPC) about a recent court order. The ruling, made by the Belgian privacy watchdog this week, was made to protect non-users from having their activity tracked by Facebook.

Facebook has agreed to comply with the court order but wrote a letter to Willem Debeuckelaere, president of the BPC, condemning the decision and claiming that tracking non-users is essential for protecting the site's security. Facebook claimed in the letter that in order to protect the security of its users, people would be required to log in or register for an account to see publicly-available content on Facebook.

How does Facebook track users and non-users? Both users and non-users of Facebook are tracked by the social network through cookies, which log the activities of web users. The main issue is with the use of a cookie called datr, which is placed on users' browsers when a Facebook page is visited or a "like" button is clicked on other sites.

In response, Debeuckelaere said Facebook's actions amounted to blackmail, according to Belgian public broadcaster VRT.

"This is not we asked for," Debeuckelaere said. "We asked them to stop following people who are not a Facebook member. Period. It seems like they are playing a game in which they are trying to corner us. As if we are the ones blocking information from the internet user."

Facebook claims that it has been the victim of more than 33,000 takeover attempts in the past month. According to Facebook, disabling the datr cookie puts the site at risk of DDoS (distributed denial-of-service) attacks and puts users at risk of having their accounts hijacked.

Failure to comply with the court order would result in a €250,000 (£177,000) daily fine being imposed on Facebook. The social network has denied that preventing non-users from accessing Facebook content was blackmail, but Belgium's federal deputy minister for privacy affairs, Bart Tommelein, agreed with Debeuckelaere's comments.

"Just because [Facebook] is a big player and the impact of their decision is large, does not mean we should give into blackmail," Tommelein said. "Everyone should abide by the privacy rules."