Pippa Middleton once shared Princess Kate's "family secret" in a book.
The Princess of Wales, 42, previously disclosed that she gifted the late Queen Elizabeth II a jar of homemade chutney for Christmas.
Kate gifted the jar to the late monarch for Christmas, and was delighted to see Queen had it on the breakfast table at Sandringham the following day.
The mother-of-three appeared in a documentary to honour the late monarch's 90th birthday in 2016, explaining what happened.
Kate and Pippa Middleton are very close sisters
Getty
The princess said: "I can remember being at Sandringham, for the first time, at Christmas.
“And I was worried about what to give the Queen as her Christmas present.
"I was thinking, 'Gosh, what should I give her?'
"I thought back to what I would give my own grandparents.
Pippa released the recipe in a book in 2012
PA
"And I thought, 'I'll make her something,' which could have gone horribly wrong.
"But I decided to make my granny's recipe of chutney."
Kate continued: "I was slightly worried about it, but I noticed the next day that it was on the table.
“I think such a simple gesture went such a long way for me and I've noticed since she's done that on lots of occasions and I think it just shows her thoughtfulness, really, and her care in looking after everybody."
Chutney is typically enjoyed with crackers and cheese
Getty
LATEST ROYAL NEWS:
Kate being at Sandringham for the first time in 2011
PA
Despite frequent requests for the recipe as a charity donation, Kate always declined, maintaining that the recipe was a family secret.
However, the full recipe was printed in Pippa Middleton's entertaining book, "Celebrate," released in 2012.
As a result, royal fans can now access the recipe, which contains marrows, apples, sultanas and dates.
These ingredients are mixed with vinegar, sugar, ground ginger and pickling spices to achieve the perfect consistency.