A body with a BMI of between 18.5 and 24.9 is classed as healthy while 25 to 29.9 is considered overweight.
A Special K spokeswoman said the new campaign was designed to encourage women to think about the positives of losing weight other than focusing on what they would look like or what size they would become.
She said: "We want to encourage a responsible attitude when it comes to body image and to show that losing weight isn't just about the way you look or a certain size you need to conform to, but more importantly about the way it makes you feel.
"The fact that we are using real women for the first time of a variety of shapes and sizes is the perfect way to encourage women to think differently about losing weight."
"The Special K girl will still be used in other advertising as she is a long-standing icon of our brand but we still insist she has a BMI of at least 21, as we only want to use healthy body images."
The new campaign - called "What will you gain when you lose" - shows the women stepping on a giant set of scales in London's Covent Garden expecting to be weighed, but instead of their weight flashing up they receive a word such as "amazing" instead.
Katie McNeil features in the new campaign after scouts spotted her while she was out shopping.
The mother-of-two, who says she has a BMI of 29, said: "I think it's really inspiring to see more realistic women and body sizes instead of size eight models all the time in advertising, which is why I was delighted to take part. Losing weight for me is about feeling great whatever size or weight I am and this campaign sums that up perfectly."