She and husband Kieran Hayler are said to be "delighted" at the news.
Katie, formerly known as glamour model Jordan, is quoted as being in "complete shock" as her fourth child Jett is only nine months old.
According to The Sun On Sunday (4 May), Katie is already six months pregnant with her fifth baby.
The newspaper claims that 35-year-old Katie conceived again last November, but only recently discovered the news after feeling unwell while on holiday in Cape Verde - where she shared lots of sun-soaked snaps with her Twitter followers.
On the idyllic break - Katie and Kieran's first holiday with baby Jett - Katie apparently suffered from painful stomach aches.
A source told The Sun On Sunday: 'It's not something they had planned this soon after Jett, but they're still delighted. The news came as a complete shock.
"Katie's already in her second trimester so has been pregnant for nearly six months without even realising it. She's due later this summer.
"Unlike her other four pregnancies, she's yet to have any cravings and is not showing much, either."
As well as Jett, her son with third husband Kieran, Katie has two children with ex-husband Peter Andre - Junior, eight and Princess Tiaamii, six, Her eldest son Harvey, 12, is the happy result of a fling with footballer Dwight Yorke.
Katie has not denied or confirmed the news.
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