Diversity may be the mantra at well-meaning movie production workshops, studio execs' away days and awards ceremonies but inequality in Hollywood is as bad as ever, a new report suggests.

The study, by the University of Southern California, examined the 100 top films in each year from 2007 to 2015 -- apart from 2011 -- analyzing more than 35,000 characters for gender, ethnicity, LGBT status and disability.   

Fewer than a third of speaking characters across the 100 top films from 2015 were female, a figure that has not changed since 2007, the university's Annenberg School for Communication and Journalism found.

Just 26 percent of characters from 2015 films analysed by the school's Media, Diversity and Social Change Initiative were from underrepresented ethnic groups. LGBT characters represented less than one percent of all speaking parts.

There was no change at all between 2007 and 2015 in the percentage of black, hispanic, Asian or other races/ethnicities in top films.

Diversity has been a major focus in Hollywood since the industry was rocked by the #OscarsSoWhite online campaign railing against the lack of ethnic minority nominees in the acting categories for the last two Academy Awards ceremonies.

But Stacy Smith, founding director of the initiative, said the study's findings laid bare Hollywood as "an epicenter of cultural inequality." 

"While the voices calling for change have escalated in number and volume, there is little evidence that this has transformed the movies that we see and the people hired to create them," she added.

Behind the camera, female directors made up just 4.1 percent of those hired on the 800 films evaluated. 

"Despite the advocacy surrounding female directors, film is a representational wasteland for women of color in this key role," said Smith.

The report also found that women were over three times as likely as their male counterparts to be shown partly nude or in sexually revealing clothing.