Followers were quick to react to the image, with one stating: "Another confirmation that Louis is a mini Kate!" But a second contended: "She reminds me a bit of baby Charlotte here. For the first time since 2015, I'm seeing some Kate in Charlotte."
WATCH: Charlotte, George and Louis ask Sir David Attenborough some questions in adorable video
A third wrote: "I usually think I see Her Majesty when I look at Charlotte but today I see Catherine. Happy Mother's Day!" while a fourth agreed: "She is the double of her!"
The post also included a photo of the cake that the three Cambridge children had baked for their mum. The caption read: "Celebrating two other special mothers today. Cake made by George, Charlotte and Louis."
Duchess Kate was seen in a throwback photo with her mum Carole
"This year Mother's Day will be different once again," Kate and William wrote in their message. "Many of us will be apart from our loved ones, but looking forward to a time in the not too distant future when we can give our mother a hug again. But for those experiencing bereavement, today may be particularly challenging.
George, Charlotte and Louis each made a Mother's Day card for Princess Diana
"Each year on Mother's Day, George, Charlotte and Louis make cards remembering their Granny, Diana, for William. Whatever your circumstances, we are thinking of you this Mother's Day."
On the back of his pictures, seven-year-old George had written: "Dear Granny Diana, Happy, Happy Mothers day. I love you very much and think of you always, Sending lots of love from George xxxxx."
On her card, Princess Charlotte, five, sweetly wrote: "Dear Granny Diana, I am thinking of you on Mother's Day. I love you very much. Papa is missing you. Lots of love Charlotte xxxxxxx."
Make sure you never miss a ROYAL story! Sign up to our newsletter to get all of our celebrity, royal and lifestyle news delivered directly to your inbox.