Ed Sheeran was ''wary'' of working with Pharrell Williams.

The 23-year-old singer's recent single 'Sing' was produced and co-written by the 'Happy' hitmaker, and it required a different way of working for the British star.

Ed explained: ''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.''

Some of Ed's new album 'x' - which is pronounced 'multiply' - was produced by the legendary Rick Rubin and the 'Drunk' star was determined to ''impress'' when the producer first expressed an interest in working with him.

He added in an interview with The Sun newspaper: ''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**t away and made me play all the songs as live and kept the raw element to the album.''