RED-CARPET correspondents weren’t the only people stunned by Hilary Swank’s knock-out Guy Laroche gown at the Oscars – so were the designers at Calvin Klein, who had spent two weeks working closely with the actress on a gown for her big night.

Instead, Swank opted for a fashion-forward navy blue Guy Laroche dress that, unlike many of the cleavage-baring gowns on display, was high-necked in front and plunged precariously low in the back – making her a true sartorial standout.

Though it was rumored that the Best Actress winner changed her mind at the last minute (as many celebs do), Swank handpicked the Laroche gown two weeks ago, in New York.

“She was doing a shoot for the cover of German Vogue, and they asked me to send some dresses,” says Georges Bully, communications manager at Guy Laroche. “When she tried that gown on, she said, ‘I want this for the Oscars.’ ”

Calvin Klein senior vice president Kim Vernon says no design house ever banks on a deal with an actress, but they were “disappointed” – especially since Swank has been the face of Klein’s underwear campaign for the past year, and wore Calvin Klein to the Golden Globes.

“We had not heard anything about Guy Laroche,” says Vernon, adding that a heads-up from Swank or her team that another dress was seriously in the running “would have been the polite and collaborative thing to do.”

But it’s not the first time Swank has disappointed a label that expected to be her first pick. “I had a deal with her to

wear a dress by Randolph Duke to a big event four or five years ago,” says Kelly Cutrone, founder of fashion p.r. firm People’s Revolution.

“At the last minute, she decided the dress was too see-through,” Cutrone says. “The stylist had gotten it into her head that Charlize Theron had just worn something like that. This stuff happens a lot.”

But despite the bruised egos, even Klein’s Vernon admits that, ultimately, “it’s just business.”

And the small, Paris-based house of Guy Laroche – hardly the household label that Calvin Klein is – couldn’t be happier.

“This is huge,” says Laroche’s Bully, adding that Swank is sending back the gown since it’s the only one and the label needs it to make samples.

The dress will be available for sale here at the end of March for just under $6,000; Swank will keep hers as a gift.

And Bully thinks Swank’s bold move is a boon for style in general.

“When you know that every fashion house is fighting [with one another] or paying to have access to stars,” he says, “something like this is just spontaneous and very simple. It’s great for fashion.”