Called Now And Then, it’s been 45 years in the making – with the first bars written by John Lennon in 1978 and the song finally completed last year.
All four Beatles feature on the track, which will be the last credited to Lennon, McCartney, Harrison and Starr.
And in a full-circle moment, it’s being issued as a double A-side single with their 1962 debut Love Me Do.
In the UK, Now And Then had its first play on BBC Radio 2 and 6 Music shortly after 14:00 GMT.
Simultaneously, the song arrived on streaming services like Spotify, Apple Music and Amazon Prime Music.
CD, vinyl and cassette copies will be available the following day. And from 10 November, the song will be included on the newly remastered and expanded versions of The Beatles’ Red and Blue greatest hits albums.
The original demo has circulated as a bootleg for years. An apologetic love song, it’s fairly typical of John Lennon’s solo output of the 1970s – in a similar vein to Jealous Guy.
It was finished in the studio last year by Sir Paul McCartney and Sir Ringo Starr. George Harrison will appear via rhythm guitar parts he recorded in 1995, and producer Giles Martin has added a new string arrangement.
“Hearing John and Paul sing the first chorus together, as they lock into the line ‘Now and then I miss you’ – it’s intensely powerful, to say the least,” said Rob Sheffield of Rolling Stone magazine.
“I cried like a baby when I heard it,” added BBC 6 Music’s Lauren Laverne. “Just gorgeous.”
The story begins in 1978, when Lennon recorded a demo with vocals and piano at his home in New York.
After his death, his widow Yoko Ono gave the recording to the remaining Beatles on a cassette that also featured demos for Free as a Bird and Real Love.
Those two songs were completed and released as singles in 1995 and 96, marking The Beatles’ first “new” material for 25 years.
The Beatles announce release of their ‘last song’
The band also attempted to record Now And Then, but the session was quickly abandoned. “It was one day – one afternoon, really – messing with it,” producer Jeff Lynne recalled.
“It has to learn what the sound of John Lennon’s guitar is, for instance, and the more information you can give it, the better it becomes,” Giles Martin told the BBC.
For Now And Then, the software was able to “lift” Lennon’s voice from the original cassette recording, removing the background hiss and the hum of the mains electricity that had hampered previous attempts to complete the song.
In McCartney’s words Lennon’s voice is “crystal clear” on Now And Then.
A 15-minute documentary broadcast on Wednesday’s The One Show, offered a startling illustration of what that means: The thin, ghostly voice of the 1970s suddenly sounds like it was recorded in Abbey Road itself.
“It was the closest we’ll ever come to having him back in the room,” said Starr. “Far out.”
