Prince William has spoken out for the first time since his nan's funeral on Monday, via a video call to a gathering in New York.
The new Prince of Wales had been due to attend the Earthshot Prize Innovation Summit and said it is due to the "saddest of occasions" that he couldn't be there in person.
William was to make the solo trip to the United States on September 21, where he would have been joined by previous winners of his £50 million global environmental competition.
In a video, recorded on Tuesday at the Windsor Estate and broadcast on Wednesday, the prince said he believed the Queen would have "been delighted to hear about" the summit.
He told those gathered: "During this time of grief, I take great comfort in your continued enthusiasm, optimism and commitment to The Earthshot Prize and what we are trying to achieve.
"Protecting the environment was a cause close to my Grandmother’s heart, and I know she would have been delighted to hear about this event and the support you are all giving our Earthshot Finalists – the next generation of environmental pioneers.
"Together in this room, are the people and organisations who will help us to achieve the Earthshot mission: to repair, restore and rejuvenate our planet within this current decade.
"That mission is not simple."
He added: "Together, we need to ensure the transition to sustainable solutions is the fastest and most endemic change the world has ever known.
"I know that by discovering, awarding, celebrating, and scaling solutions that repair and regenerate our planet we can inspire that urgent action.
"So today, as you hear the incredible stories of last year’s Finalists, I invite each of you to join me on this journey. Because this is just the start."
The Earthshot Prize was founded by William and his Royal Foundation in 2020, and in July this year the project became an independent charity, with the prince as its president.
The second Earthshot Prize awards ceremony is set to be held in Boston in December, after the inaugural event at London's Alexandra Palace last year.
Prince William and wife Kate and his two eldest children Prince George and Princess Charlotte all attended the state funeral at Westminster Abbey on Friday.
The country is still in its official 17 days of mourning for the Queen, announced by King Charles who said the royals would not be partaking in any public engagement for the next week.
