Like most of the British media, this week's OK! Magazine focusses on the Royal Baby, but instead of coo-ing over the baby prince it's drawing attention to Kate's waistline.
Katy called for all mums to boycott the magazine on Tuesday evening and so far her tweet has received more than 2,400 retweets (and counting).
Katy then tweeted a picture of her stomach two months after giving birth to show what women really look like post-pregnancy. Good on her, we say!
The hashtag #DontBuyOK is proving popular with users. Here are a selection of tweets:
Tamsin Kelly, editor of Parentdish, is equally "appalled" by OK! Magazine's cover.
"For this magazine cover line to be in UK shops on the very same day Kate left hospital with her newborn baby is contemptible."
Tamsin said that the speed of post-pregnancy weight loss among many celebrities is unrealistic for most new mums.
She said: "Women who do get back in shape fast may have an entourage of personal trainers and nutrition experts plus, and most importantly, someone to care for their baby while they are pursuing these ridiculous regimes."
"We should celebrate real women’s bodies," she added. "Women whose bodies have achieved a mini miracle – the birth of another human being.
"Real women will be recovering from childbirth, bonding with their babies, experiencing tiredness they have never known before, and extreme emotional highs and lows. They really don’t need the pressure of squeezing back into their pre-pregnancy jeans from a silly celeb magazine when managing to get out of the door with a new baby sometimes seems like mission impossible."
Well said, Tamsin.
HuffPost UK Lifestyle have contacted OK! Magazine for comment and are waiting for their response.