Britain's Prince Charles has been described as "the greatest" by Muhammad Ali.

Muhammad Ali

Muhammad Ali

The royal, who has been touring the US with his wife the Duchess of Cornwall, was welcomed to the Kentucky Centre for African American Heritage when the letter from the iconic boxer was read out.

Ali wrote: "(My wife) Lonnie and I welcome you to our hometown of Louisville, Kentucky, USA.

"We are honoured that you have come to explore, share and learn about new sustainable initiatives that are so close to your heart.

"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."

Earlier in his four-day tour, Prince Charles met with US President Barack Obama, who said he was more popular in the US than American politicians.

Obama said: "I think it's fair to say that the American people are quite fond of the royal family."

To which Charles responded: "That's awfully nice to know."

Obama then added: "They like them much better than they like their own politicians."

At which point, Charles replied: "I don't believe that."