A new Beatles song produced with a little help from artificial intelligence and including the vocals of John Lennon was released on Thursday, more than four decades after it was originally recorded as a demo.

Now And Then, first written and sung by Lennon in 1978, was finished by his fellow band members Paul McCartney and Ringo Starr... and artificial intelligence.

The track was unveiled at 3pm by Apple Corps, Capitol and Universal Music Enterprises, with a music video debuting on Friday.

McCartney, 81, announced the song's release in June, in what has been dubbed in a promotional trailer "the last Beatles song".

"It's quite emotional and we all play on it, it's a genuine Beatles recording," he said in a video on YouTube ahead of the release.

Now And Then was one of several tracks on a cassette that Lennon had recorded for McCartney at his home in New York's Dakota Building in 1979 - a year before his death.

It was given to him by Lennon's widow Yoko Ono in 1994.

Two other songs, Free As A Bird and Real Love, were cleaned up by the producer Jeff Lynne, and released in 1995 and 1996.

An attempt was made to do the same with Now And Then but the project was abandoned due to background noise on the demo.

'Kind of scary'

AI has now made that possible, although the use of the technology in music is the subject of industry-wide debate, with some denouncing copyright abuses and others praising its prowess.

After the recording was processed using the new technology "there it was, John's voice, crystal clear", McCartney said.

The two surviving Beatles finished "Now And Then" last year, including George Harrison's electric and acoustic guitar recorded in 1995.

Recording at Capitol Studios in Los Angeles, they also added Starr's drum part alongside bass, piano, a slide guitar solo by McCartney - inspired by Harrison - and more backing vocals.

Starr, 83, added the process "was the closest we'll ever come to having him (Lennon) back in the room so it was very emotional for all of us". 

"It was like John was there, you know. It's far out."

Sean Ono Lennon, the son of Lennon and Ono, said it was "incredibly touching" to hear the former Beatles working together again "after all the years that dad had been gone".

"It's the last song my dad, Paul, George and Ringo got to make together. It's like a time capsule and all feels very meant to be," he added.

The Beatles - Lennon, McCartney, Starr and Harrison - split in 1970, with each going on to have solo careers, but they never reunited.

McCartney said he agonised initially about whether they should complete the song or leave it unfinished.

"Every time I thought like that I thought 'Wait a minute, let's say I had a chance to ask John' ... I'm telling you. I know the answer would have been 'Yeah'."

Lennon was shot dead in New York in 1980 aged 40, while Harrison died of lung cancer in 2001, aged 58.

"Now And Then" will be released as a double A-side, with the band's 1962 debut single Love Me Do, and cover art by US artist Ed Ruscha.