The relationship became strained between the pair after Lennon quit the band, leading to its dissolution after the release of their final album Let It Be in 1970.
Lennon was murdered by obsessed fan Mark Chapman outside his New York apartment on 8 December 1980, aged 40.
McCartney, 78, has now revealed in conversation with Lennon’s son Sean Ono Lennon, 44, for new BBC Radio 2 documentary John Lennon At 80: “I always say to people, one of the great things for me was that after all The Beatles rubbish and all the arguing and the business, you know, business differences really… that even after all of that, I'm so happy that I got it back together with your dad.
“It really, really would have been a heartache to me if we hadn't have reunited. It was so lovely too that we did and it really gives me sort of strength to know that.”
The Hey Jude singer also reflected on what made his songwriting partnership with Lennon so special.
McCartney said: “I think what was important wasn't who was more sophisticated than the other or whatever.
“And there maybe is some truth that, musically, I had an edge because my dad had shown us some things. I'd learned the guitar chords a bit before John, but it wasn't so much that, the sophistication, it was attitudes. So my attitude would be, ‘This is what I want to do’. And then John would bring another edge to it.”
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He added: “Boy, we complemented each other. It was a bit ying yang. They say with marriages opposites attract and I think we weren't like madly opposite, but I had some stuff he didn't have, and he had some stuff I didn't have. When you put them together it made something extra, which I think was this.”
McCartney and Lennon formed The Beatles in 1960 with the late George Harrison and Ringo Starr, and they became one of the world’s most famous bands, with 13 studio albums and 17 number one singles.
Lennon’s killer Chapman recently apologised for killing the singer.
He told the parole board at New York’s Wende Correctional Facility in August this year: “I just want to reiterate that I’m sorry for my crime. I have no excuse. This was for self-glory. I think it’s the worst crime that there could be to do something to someone that’s innocent.”
He was denied parole.
The two-part documentary will mark what would have been John Lennon’s 80th birthday on Friday, 9 October.
John Lennon At 80 will air on Saturday 3 and Sunday 4 October at 9pm on BBC Radio 2.
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