Muhammad Ali hailed Britain's Price Charles as "the greatest" on Friday as the heir to the British throne wrapped up a U.S. tour with a visit to the boxing legend's hometown.

Charles, accompanied by his wife Camilla, fulfilled a packed schedule in the Kentucky city of Louisville that reflected his keen interest in agriculture and the environment.

The Prince of Wales and Duchess of Cornwall were also treated to performances at a cultural festival at the city's Kentucky Center for African American Heritage.

But the warmest welcome no doubt came in the form of a letter from Ali, a Louisville native and three-time heavyweight champion widely regarded as one of the greatest boxers of all time.

"We are honored that you have come to explore, share and learn about new sustainable initiatives that are so close to your hearts," Ali wrote to the royal couple.

"As you travel back to your homeland, we hope you know how much this city respects and admires the many contributions you have made in the world. We think you are the greatest," he added.

The letter was read to Charles and Camilla by Louisville Mayor Greg Fischer, and posted on the Instagram account of the prince's official resident Clarence House.

Ali, 73, famously nicknamed "The Greatest," has been battling Parkinson's disease for years, and in December checked into hospital with a bout of pneumonia.

Charles, 66, and Camilla, 67, spent most of the US visit in Washington, where they arrived Tuesday and met President Barack Obama on Thursday at the White House.

The prince is assuming more of the official duties that used to be performed by his mother Queen Elizabeth II, who turns 89 on April 21. She has been monarch since 1952.