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The couple - who are rumoured to be engaged - teamed up for the third single to be taken from John's sixth studio album 'Paradise Valley', which was released in August.

The 'Half Of My Heart' musician tweeted: "Who You Love is here. Proud of this song, proud of this artwork, proud of this girl", and attached the cover art for the track.

The black-and-white shot, taken by photographer Mario Sorrentil, shows John strumming a guitar while Katy's strikes a sultry pose and rests her hand on his shoulder.

John previously said about the track: "I wouldn't have brought her a song if I didn't think it was good.

"I think the power of the song protected me, certainly, I don't know about her, from it being a little too cute."

Katy also recently described their duet as "incredible", adding: "I'm so proud of it because people hear it, and they hear a different side of me."

The couple have been dating since August 2012, following Katy's divorce from comedian Russell Brand in February last year, and she reportedly wants to start a family.

A source said: "Katy brought up the idea of having a baby a few weeks ago. He was really supportive and said he'd love her to be the mother of his child. Their inner circle say it won't be long until there's happy news."