London, UK – Paul McCartney has said it was "magical" to feel as though he was reuniting with his fellow Beatles for their new song Now And Then.

Now And Then, written and sung by John Lennon and later developed by the other band members, including George Harrison, has been finished by McCartney and Ringo Starr decades after the original recording.

The track, which is described as the last Beatles song, is sourced from a Lennon demo, using technology to extricate and isolate his voice.

Describing how it came about, McCartney told BBC Radio 1: "Before John died, he was working on some songs, and Yoko [Ono, Lennon's wife] spoke to George Harrison and said 'I've got a cassette with some John songs on that he never got to finish. Would you be interested in finishing them off?'

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"So we thought about it, and we thought, 'Yeah, it would be great,' 'cause in a way, we would be working with John again, which we thought we would never be able to do.

"We worked and finished two of the songs, but we didn't get round to finishing the third one, and the third one is called Now And Then. So it was knocking around for a long time, and I kept thinking, 'There's something here, you know, we should finish this.'

"I ended up talking to Ringo, and we asked him if he fancied putting the drums on again, and then I thought, 'Well, I could up the bass a bit,' so I put the bass on again. We already had George playing guitar, and we had John on vocal – it was kind of magical doing it.

"So we ended up making it into a real record, and that's what's being released."

McCartney added: "When we were in the studio, we had John's voice in our ears, so you could imagine he was just in the next room in a vocal booth or something, and we were just working with him again, so it was joyful. It was really lovely, you know, because we hadn't experienced that for a long time, obviously, and then suddenly here we were, working with ol' Johnny."

He said he hopes the track gives fans "a loving feeling," adding: "That's often what we were trying to do with our records – we were trying to spread love. And in this one, it is very poignant.

"It's John talking about 'I miss you' and stuff like that, so ... I think 'emotion' – that would be the key word for people to take away from it, 'emotion.'"

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Asked about the technical innovation used to make the song possible, McCartney said it feels in keeping with the band's own approach to experimentation.

He said: "When there was something a little bit offbeat, technological things ... I mean, like the first time we heard a tape go backwards by mistake, we went ‘Oh, what’s that?’ and we wanted to put that on our record, whereas other people would just go ‘Oh, come on, get the tape on the right way and let’s get on with it.’

"But we always grabbed little things like that, so Peter Jackson, he's organized it so it's magic. It's very special for me to be singing with John again."