The star, 44, marked her eldest child's special day (27 May) by sharing a montage of photos of them on Instagram.
The snaps started with Harvey - who was born with multiple disabilities - as a newborn in his mum's arms, wrapped in a blanket.
Later images showed Katie and Harvey out and about together, standing at a train station, taking a dip in a pool and riding a quad bike.
"Happy 20th birthday to my gorgeous boy (man as Harvey says now) I had 20 amazing years with it just being me and Harvey, with me being his absolute constant person in his life and never letting him down," wrote Katie, who is also mum to four other children.
"He is a massive credit to me and I know he will be so excited with what I have planned this weekend and next weekend."
Sharing a single red heart emoji, she added: "I love you Harvey Price."
Fans, friends and family members headed to the comments section to send happy birthday messages to Harvey after Katie's post.
His younger brother Junior - Katie's son with former flame Peter Andre - wrote: "Happy birthday big bro, love you so much."
Former Big Brother contestant Aisleyne Horgan Wallace posted: "Happy bornday Big man️. U guys bond is so beautiful to watch, u have raised a beautiful human Katie, absolute credit to u."
Katie had Harvey with her ex Dwight Yorke.
The 20-year-old is partially blind.
He also suffers from Prader-Willi syndrome and autism.
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