George Michael, who was close friends with the prince's late mother Princess Diana, admits recording his cover of Stevie Wonder's 1972 song "You & I" marked the 'best two days of work of my life'."
And it all came about because he was feeling happy on a gorgeous spring day in London.
Premiering the song on CNN news show Piers Morgan Tonight on Friday, April 15, George Michael explained, "The idea came to me last week... that I should perhaps kinda express my happiness at this marriage, in a way, publicly."
"It was an April day that happened to be like summer here in London and I was twittering and kinda got carried away and decided I was gonna make them (William and Kate) a new track... by Friday, so I've had a very busy week."
And he admits he has always dreamed of recording the track, which was also played at the reception following U.S. leader President Barack Obama and his wife Michelle's wedding.
He adds, "When I was a child I dreamt of singing this song in my imagination... I used to dream of singing it in the school assembly hall, but I guess this is a slightly bigger deal than that."
And he hopes his free-to-download cover version is a hit among his fans and those who want a musical keepsake of the wedding - because then they'll donate to William and Kate's charity.
George Michael says, "It's not just a gift for them (royal couple), it's a gift that they'll be able to turn into many, many donations for the RoyalWeddingCharityFund.org, which is where they've asked people to give money rather than send presents."
"I would really like for any of my fans or anybody who really enjoys this track, if they're gonna download it for free... I would love them to go to Kate and William's charity site and make a donation if they're genuinely happy for these two people, which I think a lot of people are."
And George Michael has revealed Stevie Wonder has also made sure the song is a big charity earner - he has waived the rights to the song until "way after the wedding."