UPDATED, 12:48 p.m. EDT: There’s a battle shaping up in women’s undergarments. And lest you think that’s a punch line, at stake in this patent dispute is a billion-dollar industry aimed at fighting the muffin top.

Spanx, a company that’s so successful it made founder Sara Blakely the youngest self-made female billionaire in the world, filed suit March 5 against "Real Housewife of New York" Heather Thomson and her Yummie Tummie brand slimmers. The suit comes in response to a cease-and-desist letter from Thomson's attorneys on Jan. 18. The letter claims Spanx ripped off Thomson's patented three-panel design for slimming camisoles, according to the lawsuit against Times Three Clothier, LLC, which operates Yummie Tummie.

"They have copied my original idea which really was a game-changer for shapewear," Thomson told TODAY Wednesday.

In its lawsuit filed in Atlanta federal court, Spanx argues its tanks and camisoles have unspecified “significant differences” from Yummie Tummie’s design.

Since 2008 Yummie has been producing its patented three-panel Original Tank, made of cotton top and bottom panels with a slimming middle section. The Spanx items mentioned in the suit are the Top This Tank, the Top This Cami and a third, the QVC Total Taming Tank, which is no longer on sale.

Spanx issued a statement about the lawsuit on Wednesday, but declined to say when the items that are the focus of the suit first went on sale.

"Spanx designed our original shaping camisole in 2005 long before Yummie Tummie in 2008,” the statement reads. “Spanx has had countless imitators through the years. Anyone can make a claim, but it doesn’t mean it has merit. Spanx has not infringed on any valid patent, and we will continue to make fabulous products for our loyal fans."

Thomson told WWD that she first learned of the product this past November when she received an anonymous package containing the Spanx Total Taming Tank and a note saying it was on sale at QVC. "I immediately recognized it as my original Yummie Tummie tank,” Thomson told WWD. The unsigned note said Spanx was selling it at QVC. A spokeswoman for Thomson declined to comment further.

"I am still in shock and appalled at this whole situation," Thomson told TODAY.

A personal letter dated March 14 from Thomson to Blakely, posted to the Yummie website, said, “We brought this to your attention expecting you to stop. Instead you’ve chosen to sue us, no doubt thinking your massive company could intimidate ours. We have successfully enforced our design patents in the past and will continue to do so.”

“Sara, I truly expected more of you as a fellow entrepreneur,” the letter concludes.

Officials from Spanx were not immediately available to comment on the letter.