The notoriously outspoken Celebrity Big Brother star, 40, attacked the US artist when she appeared on The Graham Norton Show on Friday (20 Feburary).
In a series of tweets, Katie lashed out at the American Idol winner calling her "fat".
Katie blasted: "Jesus, what happened to Kelly Clarkson? Did she eat all of her backing singers? Happily I have wide-screen. #grahamnorton."
As her original tweet divided fans, she went on to write: "Darling, if you had a baby a year ago, that is not baby weight. It is fat. Quit calling it cute names to make yourself feel better.
"Baby weight, puppy fat, muffin top. We're so weakwilled we have to make up cute names for fat. Babies, puppies, muffins. Nope. Just fat love."
Jesus, what happened to Kelly Clarkson? Did she eat all of her backing singers? Happily I have wide-screen. #grahamnorton— Katie Hopkins (@KTHopkins) February 20, 2015
While some seemed to agree with Katie, one person tweeted: "It is never ok to slate someone for their weight or appearance. It says much more about you than it does about them."
Another said: "Thankfully Kelly Clarkson probably doesn't have a clue who you are."
As more Twitter users joined the debate, Katie added: "No one needs to agree with my views. Accepting we all think differently is the joy of life. Unless you are fat. Then clearly you're wrong.
"Let's remember boys and girls and haggard old bags, if you don't like what I say, you don't have to read it. My voice comes with choice."
Kelly, 32, gave birth to her first child - a daughter named River Rose - just eight months ago in June 2014.
She is currently promoting her new single 'Heartbeat Song'.
Unsurprisingly, Kelly has not responded to Katie's remarks...
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