Bryant made a surprise public appearance at a news conference here where he publicly apologised and begged for forgiveness from his young wife Vanessa just hours after the charges were filed.

He also denied sexually assaulting the teenager but admitted he had cheated on his wife, saying he had consensual sex with the woman.

The National Basketball Association star guard and his lawyers announced they would plead not guilty and vowed to fight the allegations which could net Bryant a life prison sentence.

The 24-year-old American was charged with one count of forcible "sexual penetration or intrusion", Colorado District Attorney Mark Hurlbert announced at a news conference.

"After much deliberation, charges were filed against Kobe Bean Bryant," Hurlbert said, announcing the much-anticipated decision to prosecute Bryant.

"As in any sex assault case this did not come easily," Hurlbert said, adding that the decision to push ahead with charges against Bryant was made after a detailed examination of evidence and testimony from witnesses.

"Then and only then did I make my decision," Hurlbert said, stressing that Bryant's case would not receive special treatment.

"The rules are the same in this case as in any other case."

News of the charge sent shockwaves through the American sports world as Bryant's clean-cut image has made him the darling of fans and led to endorsement deals with corporate giants like Nike worth 10 million dollars a year.

Hurlbert said the charges -- brought in Eagle County, Colorado, near the ski resort of Vail -- were serious and included use of physical force.

"Basically it is alleged that he had sexual penetration or intrusion, and he caused submission of the victim through actual physical force," Hurlbert said.

Bryant could face a jail term of between four years to life imprisonment or probation of between 20 years and life if convicted, said Hurlbert. The fine could range from 3,000 dollars to 750,000 dollars.

Bryant was arrested on July 4 on suspicion of sexual assault against the woman who worked at the resort where he was staying. He turning himself in to authorities.

The woman told police she was assaulted and held against her will in a room at the Lodge and Spa at Cordillera near the resort of Vail on the night of June 30.