French police have opened a criminal investigation into a raft of anti-Semitic messages posted on Twitter in a case that could test the microblogging site's refusal to mediate content.

The move follows a complaint lodged last month by an anti-racism organisation that numerous messages posted on the site had breached French law prohibiting incitement to racial hatred.

The police probe, which is still in a preliminary phase, will focus on one specific message with a view to pressing charges punishable by up to a year in
prison, judicial sources said.

France's Union of Jewish Students (UEJF) has been pressing Twitter to exercise tighter control of what appears on its site following a wave of anti-Semitic messages posted under the hashtag #unbonjuif (#agoodjew).

Examples included "#agoodjew is a dead Jew" and other offensive tweets were subsequently posted under #unjuifmort (#adeadjew).