Give Me Love singer Ed Sheeran was initially "wary" of working with Happy hitmaker Pharrell Williams.

The 23-year-old star teamed-up with 41-year-old Pharrell for his recent single Sing, and Sheeran admits he had doubts about working with the American artist.

Speaking to The Sun newspaper, Sheeran said: ''Working with Pharrell was a different way of writing a song for me. It was completely new but a good way of writing.

''He brought in the bass, hook and the drums and then I wrote all the top line.

''I was wary at first but now I am really happy about that song.''

Sheeran also spoke about working with producer Rick Rubin on his new album X, saying: ''Rick came in late. He came to gigs and expressed an interest but it was only in August when he invited me around his house in Malibu. I had my guitar with me and he asked me to play and I work best in those situations where you want to impress. He reassessed where I was with the album and came on board.

''He took all the bulls*** away and made me play all the songs as live and kept the raw element to the album.''