The 'Love Me Again' hitmaker wanted to pen the ultimate wedding track with his friend, but the 'Thinking Out Loud' singer - who also wrote the likes of Justin Bieber's 'Love Yourself' and One Direction's 'Little Things' - initially warned him he might be setting his expectations too high.
John told The Sun newspaper: ''Ed is the king of the wedding song, he said that himself.
''It's a single for me, it doesn't feature him. I emailed him, and I was like, 'Mate, I wanted to slow the music down, I don't want any gimmicks, any drops in my music, I just want some brilliant songs, so can we write something like 'Thinking Out Loud' or 'Love Yourself'?' and he was like, 'What? They're the two biggest songs, they don't come along every day', which is so true.
''But what we've got, is something up there and it's incredible. I'm really proud of it.''
The 26-year-old singer is set to undergo surgery on a brain tumour in the coming months, four years after having a similar operation, and focusing on recording new music is getting him through his medical problems.
He said: ''I'm so positive, I never look at anything as a negative.
''I'll go into my meetings and they'll tell me, 'OK, it's growing -- we'll get it out soon' and I'm like, 'Cool, OK, right, studio.'
''It's the same next year, I'll wake up from surgery and think, 'I'll go back to the studio'. It's the only thing that gets me through.''
And John is fascinated by the ''crazy'' way the tumour will be removed.
He explained: ''I find it so interesting -- it's incredible. I ask them everything they're doing all the time. These people are so incredible at what they do. They're saying people's lives.
''They go through your nose and drill through the bottom of your skull. To repair the skull they take bone from another part of your body and patch it up. It's so genius.
''Going right next to your main arteries, into your spinal cord and to the brain is crazy.''
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