The 74-year-old singer asked Robbie, 45, to collaborate for a track on his latest album, a collection of his greatest hits which have been re-recorded with the Royal Philharmonic Orchestra.
A source told The Sun's Bizarre column: ''Rod and Robbie have long been fans of each other's work and Robbie sees Rod as an inspirational mentor.
''They have been waiting for this moment for a long time. Robbie and Rod have finally managed to fit in studio time to lay down vocals for the song, which they are hoping to release later this year on Rod's upcoming album.
''It's an iconic moment for them both -- and they sound great together.''
Rod's wife Penny Lancaster and Robbie's spouse Ayda Field are also close and the pair are thrilled that their husbands are working together.
Meanwhile, Rod recently revealed he has beaten prostate cancer after a secret three-year battle.
The music legend was diagnosed with the potentially-deadly disease in February 2016 after a routine check-up, but has beaten the cancer because he ''caught it early''.
Speaking at a Prostate Project fundraising event with his with former Faces bandmates Kenney Jones and Ronnie Wood in Surrey, south east England, he said: ''Two years ago I was diagnosed with prostate cancer.
''No one knows this, but I thought this was about time I told everybody. I'm in the clear, now, simply because I caught it early. I have so many tests.
''If you're positive, and you work through it and you keep a smile on your face.
''I've worked for two years and I've just been happy, and the good Lord looked after me.''
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This article is dedicated to Caroline Flack.
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