The Rehab singer passed away in 2011, aged 27, and a posthumous compilation album, Lioness: Hidden Treasures, of unheard songs and demos curated by Mark Ronson and the Winehouse family was released later that year (11).

The singer was working on a follow-up to her hit album Back to Black when she died, and Universal Music Chief Executive David Joseph admits he has since purposefully destroyed 14 demos so the songs cannot be released.

He tells Billboard.com, "It was a moral thing. Taking a stem or a vocal is not something that would happen on my watch. It now can't happen on anyone else's."

However, Chris King, part of the team behind a new documentary, Amy, believes fans deserve to hear more of the star's unreleased material, telling Britain's The Sun newspaper, "I would recommend they find a way to release some of the stuff we listened to. There's one recording where she's getting back with (ex-husband) Blake Fielder-Civil... It's just a piano and her singing, which is such a massive emotive performance. There are songs like that and cover versions we couldn't work into the film that I'd love everybody to hear."