President Biden's climate envoy has told the BBC he hopes King Charles will continue his work on global warming.
John Kerry told Sunday with Laura Kuenssberg he would welcome the King's involvement because he "has the ability... to leverage the kind of action we need now on a global basis".
He said the monarch had "indicated" he wanted to maintain his passion for the environment "in the appropriate way".
And Mr Kerry said he hoped the King would visit the next UN climate summit.
Speaking from the US, the former Secretary of State told Laura Kuenssberg he had been invited to see the then-Prince of Wales in Scotland to discuss the climate crisis but the meeting was cancelled following the death of the Queen.
But Mr Kerry said he "very much" hopes the King will continue to push the arguments for radical action to tackle climate change "within the constitutional process".
"There's no question in my mind, that it is not a standard multilateral issue or bilateral issue, there is a threat to the entire planet, a threat to all of our nations and he understands it as well as anybody on the planet," he said.
"He's been consistently on this issue, beginning in his teenage years and carrying on in many different iterations and he's for real, believe me.
"He knows the issue inside out, backwards and forwards and he's been a leader."
Mr Kerry said the King had discussed wanting to be "relevant and try to be engaged" and that he hoped he would continue to be active on environmental issues.
"He is an enormously important convener and somebody who has the ability to be able to leverage the kind of action we need now on a global basis.
"This challenge is as significant as any threat we've ever faced," he told the BBC.
John Kerry represented the US at the COP26 climate summit in Glasgow last year
Mr Kerry was appointed President Joe Biden's special envoy for climate in January 2021 - the first time such a role has existed within the White House.
He has spoken in recent months about the impact of the war in Ukraine on efforts to limit global heating to 1.5C by 2030 - with countries facing the simultaneous challenges of sourcing alternatives to Russian gas supplies while also needing to cut their CO2 emissions.
Mr Kerry told Laura Kuenssberg the lesson from the conflict in Ukraine is that "you do not want to be a prisoner of your energy source" adding that Europe "wants to liberate itself" from its dependency on Russian gas.
He added it would be "terrific" if the King could come to the COP climate change summit in the Egyptian resort of Sharm el Sheikh in November - adding that he thought His Majesty would like to be able to attend.
Mr Kerry said that he thought next gathering would see a "raising of ambition" by countries which had been unable to sign up at the COP26 summit in Glasgow last year to steps to limit the planet's temperature increase to 1.5C.
"We can get there but it's going to take a gigantic effort moving forward," he said.