Pittsburgh Steelers quarterback Ben Roethlisberger has been suspended by the NFL for six games without pay.

This was despite charges of sexual assault against the 28-year-old being dropped.

Last week prosecutors elected not to pursue charges against Roethlisberger after a 20-year-old college student accused him of sexual assault. Roethlisberger denied the charges.

Despite that, NFL commissioner Roger Goodell has determined that Roethlisberger has violated the league's person conduct policy, leading to the suspension.

"The Personal Conduct Policy makes clear that I may impose discipline 'even where the conduct does not result in conviction of a crime' as, for example, where the conduct 'imposes inherent danger to the safety and well being of another person'," Goodell wrote in a letter to Roethlisberger.

"As the District Attorney concluded, the extensive investigatory record shows that you contributed to the irresponsible consumption of alcohol by purchasing (or facilitating the purchase of) alcoholic beverages for underage college students, at least some of whom were likely already intoxicated. There is no question that the excessive consumption of alcohol that evening put the students and yourself at risk.

"The Personal Conduct Policy also states that discipline is appropriate for conduct that 'undermines or puts at risk the integrity and reputation of the NFL, NFL clubs, or NFL players.' By any measure, your conduct satisfies that standard."

Goodell left room to reduce the suspension to four games if Roethlisberger demonstrates good behaviour prior to the start of the season.

The Steelers have signed Byron Leftwich to provide cover for Roethlisberger, who has led the team to two Super Bowl titles since debuting in 2004.