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Muhammad Ali welcomed the Prince of Wales and Duchess of Cornwall to his home city of Louisville, describing Charles as "the greatest".
The sport star's words came in a letter rather than in person as he was not well enough to join leading figures from the city at a cultural greeting for the royal couple, who were in the city on the final day of their US tour.
Instead, Louisville's Mayor Greg Fischer welcomed the royal couple by reading them Ali's letter:
The Duchess of Cornwall has paid a visit to the home of the world famous Kentucky Derby where she met two unlikely entrants.
As part of her trip to Lousiville, Camilla was at the Churchill Downs racecourse to celebrate the work of the Brooke, a worldwide equine welfare charity of which she is president.
Whilst there she met a pair of donkeys and gave them treats after "giving them a good scratch".
The visit came on the final day of Prince Charles and Camilla's four day trip to the US.
Prince Charles has said there is "awful lot to worry about" in relation to the world's environmental resources and wildlife.
He said his conservation efforts were driven by a need to leave future generations a world "which isn't even more destroyed and damaged and dysfunctional than it need be".
He listed his areas of concern from the continuing destruction of rainforests, to the threat to endangered animals like rhinos, elephants and tigers and the need for sustainable cities.
Charles said: "The world has looked to the United States for leadership in so many challenging circumstances in the past.
"However, today we are faced by truly exceptional challenges and threats - a veritable 'perfect storm' which, if not met by strong, decisive and far-sighted leadership, could overwhelm our capacity to rectify the damage and thereby destroy our grandchildren's future inheritance."
His comments came after he was presented with the exceptional leadership in conservation honour, from the International Conservation Caucus Foundation during a ceremony in Washington.
The Prince of Wales said during his acceptance speech that he realised he had been coming to the US for the past 40 years.
"Look what it's done to me," he joked before saying his only anxiety about the evening was "that I might quite possibly send my wife to sleep during my speech."
Prince Charles has been awarded the Teddy Roosevelt International Conservation Award for his "extraordinary" conservation leadership.
The Prince of Wales accepted the award as he attended the International Conservation Caucus Foundation dinner.
Prince Charles attended a reception at the British Ambassador to the US's residence in Washington after a busy day of events across the city.
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