It's sibling devilry, as sister battles sister in the season's hottest behind-the-scenes brouhaha

Something downright nasty has been percolating on the Charmed set--and I'm not talkin' bat wings or toad heads.

No, this witchy potion smacks of ingredients far more familiar to the world of Aaron Spelling: two divas, one script and a knock-down-drag-out catfight.

For months, there have been whisperings about tension between Shannen Doherty and Alyssa Milano, but until now, no one close to the show would confirm or deny. However, in the past few weeks, as the tension between this diva-ish duo has escalated into a witchy war vicious enough to put the future of the show in peril, the stench from the set is seeping out in full force.

Among the juiciest morsels: Alyssa is so fed up with Shannen's 'tude she has threatened to leave unless something changes. The producers are doing some serious ass-kissing, trying everything in their power to get their Charmed girl to stay.

See, the show's founding premise, "The Power of Three," demands that the three sisters work together to fight evil, which has painted the writers into an ugly corner. None of the girls can leave without the show crumbling.

Although Shannen and Alyssa have repeatedly requested as few scenes together as possible, the producers have no choice but to make the archenemies share the screen. And when they must, my sources say they show up, run through their lines and storm off.

Why such ugliness? Well, from what I hear, it all began with the show's very first publicity shots, where Shannen insisted on being placed in the center, or above the others, at all times. (One source close to the WB even hinted it was written into her contract.) It seems that while Alyssa and Holly Marie Combs were told it was an ensemble show, Shannen has claimed it as her own since day one.

Then there's the issue of wardrobe. Although all three girls wear fab threads, it's Shannen who gets the biggest chunk of the wardrobe budget, accepting nothing less than Prada, Dolce & Gabbana and the like. The wardrobe folks spend most of their time finding and reworking duds for the queen bee.

As for her royal 'tude, let's just say that during a recent taping of the "Wrestling with Demons" episode, one of the pro wrestlers ended up calling the witchy woman a bitch, which later--behind closed doors--garnered wild applause from the tongue-tied crew.