Prince George may not have been able to attend the British girl's sixth birthday party last week, but Prince William and Kate Middleton were "very touched" by the invitation and sent "warm thanks and best wishes" to the child's mother.

According to multiple reports, the warm welcome came in the form of a heartfelt letter written by staff for the royal couple. The mother, under the username @xxAimeeRoxyxx, posted a photo of the letter on Twitter with the caption, "This is a wonderful thing to keep," according to Express UK.

According to sources who follow the now private account, the letter reads, "The Duke and Duchess of Cambridge have asked me to thank you for your letter inviting Prince George to his 6th birthday party. I apologize for the delay in responding. Their Royal Highnesses were very grateful for the kind invitation.

The invitation was sent several months ago, but given the family's move to Adelaide Cottage in Windsor, they unfortunately had to decline the invitation. The letter continues. After careful consideration of the possibilities, we are very sorry that Your Highnesses have reluctantly felt obliged to decline. Nevertheless, we hope that you enjoyed your birthday."

The letter concludes. "The Duke and Duchess of Cambridge are very touched that you have taken the trouble to write to their son, Prince George. Their Royal Highnesses have asked me to express their sincere thanks and pleasure."

The Duke and Duchess of Cambridge have also written to the Duke and Duchess of Cambridge to thank them for their kindness.

Prince George celebrated his own birthday last month, a testament to the young prince's rapid development; sandwiched between a platinum jubilee in June and a birthday in July, it was a busy summer for the royal family. And as the end of the month approaches, the three children will be attending a new school in the Berkshires beginning in September.