The 'Day I Died' singer was given advice from the pop legend - who helped him secure a management deal - when starting his career but insists he was never over-awed by the 62-year-old musician's worldwide celebrity status.
Jack said: "I didn't really see him as anything beyond a guy I met and had a little chat to. At the time, to be honest, I wasn't that aware of the breadth of his songwriting and hadn't heard that much of it when I was young."
Since finding success, Jack has forged a firm friendship with 'Fear' singer Lily Allen and considers them both very similar artists.
The 34-year-old star explained: "There have been comparisons made between us, so I guess these things are inevitable. When I first heard her, it was like listening to myself."