"Having a smaller family reduces our impact on the Earth, and provides a better chance for all our children, their children and future generations to flourish on a healthy planet.
"We commend the Duke and Duchess for taking this enlightened decision, and for affirming that a smaller family is also a happy family."
Prince Harry and Duchess Meghan, 39, will be given the award on Sunday, to mark the United Nations' World Population Day.
Harry, 36, first spoke about the couple's family plan in an interview with chimpanzee expert Dr Jane Goodall in Vogue magazine in 2019.
Speaking about his concerns for the future of the planet, Harry said: "I've always wanted to try and ensure that, even before having a child and hoping to have children …," before Goodall interjected: "Not too many!".
He replied: "Two, maximum! But I've always thought: this place is borrowed. And, surely, being as intelligent as we all are, or as evolved as we all are supposed to be, we should be able to leave something better behind for the next generation."