It is the first time criminal charges have been brought against the veteran entertainer
US star Bill Cosby has been charged with sexually assaulting a woman in Pennsylvania more than a decade ago.
It is the first time criminal charges have been brought against the veteran entertainer in the wake of a slew of allegations over his behaviour.
The announcement sets the stage for a Hollywood celebrity trial. It could potentially result in prison for the 78-year-old TV comic in the twilight of his life.
In bringing the case, Montgomery County district attorney Risa Vetri Ferman overruled her predecessor, who declined to charge Cosby in 2005 when Temple University employee Andrea Constand first told police that the comic drugged her and assaulted her at his mansion in suburban Philadelphia.
Cosby was charged with aggravated indecent assault and was to be arraigned in the afternoon.
The TV star acknowledged under oath a decade ago that he had sexual contact with Constand but said it was consensual.
Charges were announced just days before the 12-year statute of limitations for bringing charges was set to run out.
Prosecutors reopened the case over the summer after testimony was unsealed in Ms Constand's related civil lawsuit against Cosby and dozens of other women came forward with similar accusations.
"Reopening this case was not a question. Rather, reopening this case was our duty as law enforcement officers," Mr Steel said.
He urged any other alleged victims to come forward as well.
Cosby is facing a long list of legal problems, including defamation and sex abuse lawsuits filed in Boston, Los Angeles and Pennsylvania.
But as for criminal charges, many of the alleged assaults date back decades, and the statute of limitations has expired in nearly every case.
Ms Constand, who now works as a massage therapist in her native Canada, is ready to face Cosby in court, her attorney Dolores Troiani said.
"She's a very strong lady," Ms Troiani said. "She'll do whatever they request of her."