The controversial journalist claimed the 34-year-old didn't care about her looks, after she was pictured in an unflattering yellow dress at Royal Ascot on Tuesday.
The mother-of-one's outfit led to speculation she was expecting, however a spokesman stated: "Zara is not pregnant - her plans remain the same in terms of preparation for Rio 2016."
Celebrity Big Brother star Katie, 40, slammed Zara for looking "six months" along, while also poking fun at her husband, Mike Tindall.
Writing in The Sun today, she stated: "There is something I love about Zara Tindall.
"Wearing a super-tight yellow dress that makes you look six months pregnant while K-Mid [Kate Middleton] is back to a size six just six weeks after giving birth is a great way to highlight the fact you don’t care what anyone thinks."
As well as the rugby star, Zara's mother, Princess Anne, was also mentioned in the cruel piece.
Katie added: "If you marry a man with a nose where his ear should be and your mother is Princess Anne, I suspect looks aren’t your top priority and laughter is."
Meanwhile Olympian Zara stuck to a safe black dress last night, for the launch of her Calleija jewellery collection.
The star did however flash her pins in the sleeveless number, which came complete with a flared skirt and modest neckline.
She polished off her attire with a very chic pair of cut-out heels and a grey clutch, while wearing her tresses down in a loose wave.