AMERICAN IDOL alum gay hate fest ignites as ADAM LAMBERT trashed CLAY AIKEN on ANDY COHEN’s yak fest.

A LONGSTANDING bitter­ness between “American Idol” alums Clay Aiken and Adam Lambert is bubbling over after Adam trashed Clay during a recent appearance on Bravo’s “Watch What Hap­pens Live.”

On May 24, Adam, 30, told talk-show host Andy Cohen he “couldn’t think of any” nice things to say about Clay during a game segment of the show.

 It’s just the latest swipe in the simmering feud that started when Clay blogged in 2009 that he thought one of Adam’s “AI” performances was “contrived, awful and slightly frightening.”

According to a source, Adam “absolutely despises” Clay, 33,  for taking so long to admit he is gay.

The Season 2 “AI” runner-up didn’t come out of the closet until five years after his “Idol” debut, while flamboyant Adam publicly embraced his sexu­ality shortly after he won second place on Season 8.

Adam’s riding high these days as his latest album, “Trespassing,” debuted at No. 1 on the charts.

Clay’s latest accomplishment was runner-up to Arsenio Hall on this season’s “Celebrity Apprentice.”

After his latest dig at Aiken, the source said: “The next move is up to Clay.”