John Kerry said in an interview it would be "terrific" if the King were able to be there, adding that he has been "a terrific leader on this issue".

As Prince of Wales he had planned to go to November's COP27 conference.

But after ascending the throne he decided not to attend on the advice of Prime Minister Liz Truss.

Liz Truss has announced she will resign, which means there will now be another leadership election to decide who becomes the next Conservative leader and prime minister.

The contest to replace her is due to be completed by 28 October.

Secretary Kerry said it would ultimately be up to "whatever government is in place" to make the decision about attending the climate conference, which runs November 6-18.

As Prince of Wales, Charles spent decades campaigning on environmental causes.

Now he is king he is subject to different rules; the monarch is obliged to remain politically neutral.

Secretary Kerry said he did not consider championing climate action as "political".

"It's a generic broad, based, existential issue for the world, and his leadership has been very important", he said.

Last November - as the Prince of Wales - the King travelled to Egypt with the government's blessing to urge the Egyptian administration on its efforts, meeting President Abdul Fattah al-Sissi.

Egypt, the hosts of the summit, warned the UK not to backtrack on the global climate agenda following the announcement that King Charles would not attend.

Secretary Kerry also told BBC News on Friday that he had also noted what he called "decisions that raised some questions" by the current government.

He said the US is concerned about a number of countries that appear at risk of rolling back on their climate commitments.