JESSICA SIMPSON’s clothing line has hit the $1 BILLION sales mark – and that’s got designing twins MARY-KATE and ASHLEY OLSEN in a  snit!

A source says the former “Full House” stars compete with Jessica be­hind the seams, and while the pint-size blondes also head a billion-dollar empire, they’re bitter over 33-year-old Jessica’s mass-market appeal.

“Mary-Kate and Ashley believe Jessica’s fashions are ‘low rent,’” a source told The ENQUIRER.

“They think she has little fashion sense and caters to the lowest common denomi­nator just to make a buck.”

The Jessica Simpson Collection is sold in more than 650 department stores and includes bags, boots and shoes, fragrance, jewelry and sunglasses.

But the Olsens, 27, say there’s no comparison between her product line and their fashion empire which includes, a beauty line called Mary-Kate and Ashley: Real Fashion for real girls, luxury label The Row, Elizabeth and James, and Olsenboye.

The Row’s crown jewel, for example, was a $39,000 alligator backpack!

“Mary-Kate and Ashley are regulars at all the fashion shows in New York, Paris, London and Milan, and dress way ahead of the curve,” said the source.“To them Jessica is flogging flannel and burlap to banjo pickers. They think her Daisy Duke view of fashion is anything that will bring in a dollar. On the other hand, they have their fingers on the pulse of fashion worldwide and work hard at creating sophisticated designs.”

A rep for the Olsens denies that they are feuding with Simpson, adding that they “understand hard work” and are “thrilled for anyone’s success.”