Gabrielle Union says her 'America's Got Talent' exit was the ''hardest part'' of her time in Hollywood.

The 47-year-old actress filed a complaint about ''toxic'' behaviour on the set of the NBC talent competition earlier this year, following her decision last year to quit the show after just one season on the judging panel because of the alleged racial bias and discrimination she faced.

And now, Gabrielle has admitted the situation was ''so heartbreaking and so unnecessary'' that she now views it as the most difficult part of her career to date.

When asked for her toughest career moment during an interview with USA Today, she said: ''Probably the 'AGT' ... it all was so surprising and so heartbreaking and so frustrating and so unnecessary. That would be probably the hardest part.''

The 'Being Mary Jane' star filed a discrimination complaint against NBC, Simon Cowell, FremantleMedia and Syco Entertainment in California's Department of Fair Employment and Housing in June this year.

However, the case has since been closed as she instead requested an immediate right to sue, which she was granted.

Her official complaint came after NBC previously conducted an independent investigation into Gabrielle's allegations, but she was unhappy with how it was handled.

She said at the time: ''They were like, 'We're going to commission this independent investigation.' Well, silly me. I thought independent was independent, but when NBC and Fremantle and Syco pay for that investigation, they control it.

''They turn over what they believe to be inflammatory things, or things that are not advantageous to me, over to the head of NBC, Paul Telegdy, who uses those things that he thinks are smoking guns to shoot down my claims.

''He then threatens my agent [by saying,] 'Gabrielle better watch who she calls a racist' in the middle of an investigation about racism and discrimination. This is what's happening from the top of the company.''

NBC previously insisted it was ''categorically untrue'' that any threats had been made towards the star.

They said in a statement: ''We took Ms. Union's concerns seriously, and engaged an outside investigator who found an overarching culture of diversity on the show. NBCUniversal remains committed to creating an inclusive and supportive working environment where people of all backgrounds are treated with respect.''