In a touching show of affection to his mother, James left a comment with three heart emojis.
Carole looks incredible in a light blue jumper, skinny jeans and brown suede boots in the photo.
Kate’s mother jokes about it being up to her husband Michael, 71, to help her celebrate her big day this year.
She says in the caption: “If like me, your birthday falls during the cold, dark days of winter, your loved ones can still make the day full of surprises.
“Our customers have been transforming their surroundings with our sparkly backdrops and bright balloons for small ‘at-home’ parties for their household.
“It may not be how we imagined we would be celebrating, but a little effort can still lift the spirits and bring some joy.
“This year it’s down to my husband to help me mark my day – no pressure!”
Many other Instagram users left comments wishing Carole a happy birthday.
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Another added: “Beautiful photo and lovely message.”
A fifth said: “If Carole Middleton says it, I believe it!”
One more commented: “Happy Birthday. My winter birthday celebration is up to my husband as well.”
The Middletons, who have three grown-up children, live at Bucklebury Manor in Berkshire.
Carole set up Party Pieces from a garden shed in 1987 after not being able to find what she wanted on the high street.
In 1989 it moved to a new headquarters in Hungerford.
The family business’s growth led to another move in 1993 to Childs Court Farm in Yattendon.
In 2019, Party Pieces celebrated its 30th anniversary.