Dan Smith and co brought the Scottish star out on stage as a special guest for a rendition of their hit song 'Joy' at Glastonbury last month and he also opened for the band across the UK and Europe earlier this year.
Dan - who has also written for Craig David, Foxes, and Tears For Fears - has revealed they managed to get the 'Grace' singer in their makeshift studio on the road.
Speaking to The Sun newspaper's Bizarre column, he said: ''We wrote together on tour. It was really nice.
''I've been doing quite a lot of writing with other people.
''We'd always have a studio set up backstage so we wrote a song together, which is awesome.
''He sang it and it sounds incredible.
''His voice is way more impressive and powerful than mine. I love the song, I think it's wicked.''
The 'Happier' hitmaker also revealed that he turned down the chance to work with Ed Sheeran because was ''a bit too shy'' at the time.
He said: ''Years ago he invited me to his house to write for a week and I think I was a bit too shy to say yes.
''I'm kicking myself now. Ed's lovely, he's one of the most genuine people in the music industry.''
Dan recently admitted he feels like an ''imposter''.
The 'Pompeii' singer insisted he isn't worried about the group's experiments with different genres on their latest 'Doom Days' because he's ''comfortable'' with where they are musically, even though he still feels like he leads a ''double life''.
He said: ''We wanted to take the listener through different moods and moments and I think we are finally really comfortable with where we are at.
''Don't get me wrong, I still feel like an imposter and I still think we lead this double life where we go off on tour, play these shows and then go home and everything is back to normal.
''But we've never really cared about genre. It is irrelevant to us and so on our albums or our mix tapes we've done everything from pop to hip-hop to guitar music. We do whatever we want to.''
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