“ was able to extricate John’s voice from a ropey little bit of cassette,” McCartney said. It was this process that allowed McCartney to “duet” with Lennon on his recent tour, including at last year’s Glastonbury festival, and for new surround-sound mixes of the Beatles’ Revolver album last year.

For the documentary, dialogue editor Emile de la Rey used custom-made AI to recognise the Beatles’ voices and separate them from background noise.

The idea to use AI to reconstruct the demo came from Peter Jackson’s eight-hour epic, Get Back. The lyrics, which begin “I know it’s true, it’s all because of you / And if I make it through, it’s all because of you”, are typical of the apologetic love songs Lennon wrote in the latter part of his career. It was largely recorded on to a boombox as Lennon sat at a piano in his New York apartment. The demo was one of several songs on cassettes labelled “For Paul” that Lennon made shortly before his death in 1980, which were later given to McCartney by Lennon’s widow, Yoko Ono. Though McCartney did not name the song, it is likely to be a 1978 Lennon composition called Now and Then. “We just finished it up and it’ll be released this year,” he told the Radio 4 Today programme on Tuesday.

Tell me why beatles