Karim Benzema Charged with Extortion ... In Sex Tape Blackmail Case

Things have gone from bad to worse for soccer superstar Karim Benzema ... who has now been hit with formal charges for allegedly conspiring to extort a teammate over a sex tape.

The 27-year-old Real Madrid star was arrested Wednesday in France -- with investigators claiming he had a role in trying to extort his French national teammate Mathieu Valbuena.

Benzema spent the night in a cell in Versailles ... and now, he's been formally charged with complicity in an attempt to blackmail and conspiring to commit a criminal act.

The sex tape in question was reportedly shot on a cell phone -- officials say Benzema was part of a plot to extort money out of Valbuena and in exchange they wouldn't publish the tape.

If Benzema is convicted, he faces up to 5 years in prison.