The Duke and Duchess of Cambridge have been urged to refrain from having a third child in a bizarre open letter written by family planning organisation Having Kids.

Progressives at the organisation worried about climate change and social justice favour smaller families and have created an online stir for advising Prince William and Kate Middleton to stick with two children on the grounds that a crowded nursery damages "sustainability and equity".

They wrote in the 25 July letter: "Melting glaciers could affect the North Atlantic circulation, completely changing the United Kingdom's weather.

"All of us — especially public figures — should plan our families with the future environment in mind, producing a smaller and more resilient populace capable of thriving in that environment."

They wrote the letter to the British royals to create awareness and use their example as an influence.

It was signed by Having Kids Executive Director Anne Green and President Carter Dillard, and the letter has received a huge amount of backlash since. After royal fans and members of the public criticised the letter, Green has explained why they wrote to the royals and defended their original argument that William and Kate should adopt the Fair Start family planning model in a follow-up letter.

She said: "It's not about altruism or charity. It's about working together to help the actual people who will comprise the future, our future. Many in the past have given their lives to protect this world from various threats. The least we can do is plan our families in a responsible and sustainable manner."

The company also argued against the point made that non-British companies should not interfere with the British Royal Family's affairs. Having Kids say that climate change is a "global" problem which does not recognise borders, stating: "It's cognitive dissonance to say that choices which so profoundly affect our world are none of our business."

Just last week, IBTimes UK reported that Prince William wasn't so keen on expanding his brood, despite Kate being rumoured to want a third child.

Kate joked to one of our royal insiders that William was not particularly eager regarding a third baby approximately 18 months ago.

She said: "If I got pregnant now... I think William would be out the door!"

It was said in a laughing manner and by no means rules out another child for the young royals, but there are other indicators as to why Kate and William might stick to two children, such as busy schedules as working royals and Prince Philip's encouragement to only have two children.