Duchess Catherine has been urged not to have any more children for the sake of the environment.
The 35-year-old royal already has Prince George, four, and Princess Charlotte, two, with her husband Prince William, but has been implored by Having Kids to refrain from having more children.
The American organisation, which is based in San Francisco, has called on the royal couple to serve as role models for the public, saying that "large families are not sustainable".
In a letter, Having Kids explained: "Family planning has the most potential for mitigating climate change and its impacts."
The group subsequently warned of the "impacts" of over-sized families, saying: "More severe flooding - both from heavier rains and coastal flooding; deadlier heatwaves; increases in diseases such as malaria; and wildlife extinctions."
Meanwhile, it was reported last month that the royal couple have banned their children from using iPads.
Duchess Catherine and Prince William don't want Prince George and Princess Charlotte to become reliant on technology for entertainment and want to encourage them to use their imaginations.
A source said: "They're very much seen as Mummy and Daddy's toys, not for children.
"As two people who grew up without gadgets for entertainment themselves, William and Kate are firm believers in toys, outdoor play, and encouraging an active imagination. They're very much a normal family."
In fact, Duchess Catherine recently spoke of her joy at seeing her children fall in love with nature.
The royal made reference to her kids in a speech she gave to mark the reopening of Hintze Hall at the Natural History Museum in London.
During her speech, the Duchess explained: "Like many of you here tonight, I remember visiting the Natural History Museum as a child, and being inspired with a love of nature.
"And now, as a parent myself, I am experiencing the joy all over again with my own children, who adore coming here, and it is not just to see the T-Rex mind you.
"Who could fail to be inspired standing here in this wonderful hall."