On the front on the cards, which were sent out to all members of the public that had included an address with their message of congratulations, is a picture of the family standing on the step of the Lindo Wing just one day after Kate gave birth.
"Their Royal Highnesses have been overwhelmed by all the wonderful messages they have received and send you their warmest thanks and best wishes," reads the card.
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Zoe Hall from Brighton was among the fans to receive a card.
"I sent the card the day the birth was announced in July and I am amazed I got a reply so quickly," said the royal fan.
"I am absolutely thrilled and will definitely be getting it framed. I wanted to send a card as I was so excited about the announcement.
"It's nice that they are recognising people's interest in them."
A Kensington Palace spokesperson said, "The Duke and Duchess of Cambridge are very grateful for the support and goodwill being expressed at this happy time and wanted to acknowledge the letters and gifts they have received."
The cards show Kate wearing a bespoke cornflower blue polka dot dress by Jenny Packham, cradling her newborn baby while Prince William waves to all those gathered.