Prosecutors in the US have charged comedian Bill Cosby with aggravated indecent assault over an alleged incident in 2004.

It is the first time Cosby, 78, has been charged with any offence after months of accusations.

Andrea Constand says she was drugged then assaulted at the Philadelphia home of the former US television star.

Dozens of women have accused him of sexual assault, dating back to the 1970s, but he has denied wrongdoing.

Cosby made no comment as he arrived at court to be formally charged. He did not enter a plea at the hearing where a judge set bail at $1m.

Later, his lawyer said in a statement that he would "mount a vigorous defence against this unjustified charge".

Earlier this month Cosby opened a lawsuit against seven of the women, accusing them of defamation.

It is the second time there has been an investigation into Constand's complaint against the actor, who won acclaim for playing a family doctor and model father in The Cosby Show.

In 2005, after no charges were brought when she first made the accusation, she sued him and settled for an undisclosed sum.

He has previously said under oath he had a consensual sexual encounter with her.

Cosby befriended Constand through her job at Temple University and she came to think of him as "a mentor and a friend" before he allegedly made two sexual advances that were rejected, said prosecutors.

Documents from her lawsuit against the star were sealed until this summer, when damaging testimony was revealed as more women came forward.

The 12-year statute of limitations was only days away from expiring, in January 2016.

Kevin Steele, a district attorney in Montgomery County near Philadelphia, said his office had reopened the Constand case after "new information came to light".

The prosecutors re-interviewed witnesses, examined evidence from the civil case and spoke to other alleged victims before they decided to file criminal charges, Steele's office said in a statement.

In testimony seen by the New York Times and published in July, Cosby admitted he had given women a sedative, Quaaludes, when he wanted to have sex with them, but not without their knowledge.

At the height of his fame from The Cosby Show, the comedian was the highest paid entertainer on US television.