A Belgian watchdog on anti-Semitism warned that it was preparing to take legal action against the online propagators of an anti-Semitic joke about the Holocaust.

The Belgian League against Anti-Semitism, or LBCA, issued the threat regarding a photograph of an oven with paper money inside bearing the caption “The Jew trap is set.” The photo surfaced earlier this month on Twitter and later Facebook.

“This disgusting and unacceptable new joke has a double insult,” said LBCA President Joel Rubinfeld. “It alludes to the anti-Semitic stereotype that Jews are greedy and to the crematoria that Nazis used to burn Jews during the Holocaust.”

The tweet and the Facebook post have been removed, though it is unclear whether the users or the companies removed them.

One of the photo’s early disseminators was a Twitter user from Belgium using the handle Simree Dikule. Thousands of additional users re-tweeted the picture, which was posted also on Facebook by a student of the Bracops Lambert High School near Brussels, the La Capitale daily reported Tuesday. The student, who was identified only as “M.,” wrote on Facebook, “It’s racist, but it’s funny.”

Last year, a French court forced the California-based Twitter social network to divulge information on users who had disseminated anti-Semitic jokes. The UEJF French Jewish student union sued Twitter after the hashtags #unbonjuif (“a good Jew”) and #unjuifmort (“a dead Jew”) became hugely popular because they were used in what Le Monde termed “a competition of anti-Semitic jokes.”

Hashtags are labels used to index tweets on a particular topic.

Twitter argued that as an American company, it adhered to US laws on hate speech, which are far more liberal than those of France and many other European countries. But the French court ruled that the French blogosphere and online interaction between French residents constituted French public space, where French legislation against hate speech applies.