GET ready for the battle of the heavyweight sitcom champs!

“Fat Actress” Kirstie Alley is convinced the “new” “Mike & Molly” star Me­lissa McCarthy is trying to butt in on Kirstie’s role as reigning queen of the plus-size funny ladies, say sources.

The 5-foot-8 former “Cheers” star, who’s pushing 220 pounds, is fed up with 5-foot-2, 201-pound Melissa. And it’s all because of a plot twist in Melissa’s sitcom.

A rep for Kirstie denied that she is annoyed with McCarthy and pointed to a complimentary tweet Kirstie posted about McCarthy more than a year ago.

Indeed, Kristie, 62, used to be fine with 43-year-old Melissa… until CBS began touting the show’s Season 4 premiere, in­serting the word “new” into the title and introducing a plot twist in which Molly leaves her teaching job of 10 years and drunkenly car­ries on about pursuing a career in musical theater.

“Kirstie was flabbergasted,” said the insider. “She feels like the sudden plot change is a slap in the face because in her new sitcom (debuting on TV Land in early December), Kirstie is a musical theater star.

“She’s not taking the adjust­ment to Melissa’s character lightly. She now sees Melissa as her com­petition – and has let her know it.”

Although Kirstie works in a business that idolizes youth and beauty, she’s proud of taking over the chubby sitcom queen crown from Roseanne Barr when her show, “Roseanne,” went off the air in 1997.

“With vehicles like ‘Veron­ica’s Closet’ – in which she gained a lot of weight – and ‘Fat Actress,’ Kirstie felt she became everybody’s funny fat lady,” said the insider. “That is, until Melissa came on to the scene.”

Along with her TV show, Melissa has also wowed big-screen audiences in the movies “Bridesmaids” and “The Heat.”

“Now Kirstie believes that Melissa has crossed the line with the new plot line on her show,” continued the source.

“Kirstie is as sweet as pie to most people in the industry, but when she feels betrayed she can be as nasty as they come.”

Added a close source: “Me­lissa is feeling Kirstie’s wrath, but the battle is one-sided. She feels it’s the more the merrier as far as flabby funny ladies are concerned.”