After hurling a vicious gay slur at one of his co-stars, Isaiah Washington has become an outcast on the “Grey’s Anatomy” set.

Co-star Katherine Heigl has asked producers to ban him from the set when she’s working – and other cast members shun him unless they’re filming a scene, sources tell The ENQUIRER.

Now the 43-year-old actor is so desperate to keep his job that he’s promised producers he’ll go into therapy.

That’s the untold story behind the shocking incident – first reported by The ENQUIRER – that’s turned the “Grey’s” set into Hollywood’s most dysfunctional workplace.

“No one is happy working with Isaiah. The cast and crew feel like the set is a war zone,” an insider revealed.

“The actors have to tiptoe around and watch what they say and do – and that’s not conducive to creativity.”

The problems began when Isaiah – who plays Dr. Preston Burke – choked co-star Patrick Dempsey – who plays Dr. “McDreamy” Derek Shepherd – during a violent on-set brawl on Oct. 9.

At one point in the melee, Isaiah yelled at Dempsey, “I’m not your little faggot like T.R.”

T.R. Knight plays mild-mannered Dr. George O’Malley on the hit ABC show.

The ENQUIRER broke the story without naming Knight, but he later decided to come out of the closet.

The cast managed to smooth things over – until they took the stage at the Golden Globes on Jan. 15 after winning the award for Best TV Drama.

During a question- and-answer session, Isaiah stepped forward and announced: “I did not call T.R. a faggot. Never happened, never happened.”

Since then, “the entire cast is treating Isaiah like he has leprosy,” a set source tells The ENQUIRER.

Pick up this week’s issue for all the details!