The Beatles' last song is on track to become their 18th number one single.
Now And Then was written and recorded by John Lennon in New York in the late 1970s, before being developed by the other band members, including George Harrison, in 1995.
Lennon's wife Yoko Ono had given the recording to the remaining Beatles the previous year.
But technology at the time was more limited and the band were unhappy with the sound quality.
Now, however, with the help of AI, Sir Paul McCartney and Sir Ringo Starr have finished it, more than four decades after it was started.
Filmmaker Peter Jackson's recent Beatles documentary used audio restoration technology. That allows music and vocals to be isolated, a technique that was used on Now And Then.
Jackson also directed the new music video for the song which was released on Friday.
In its opening weekend, the song has outsold the rest of the top five combined, according to the Official Chart Company.
It is the Fab Four's first chart-topper in 54 years, the last being The Ballad Of John And Yoko in 1969.
Previous number ones include All You Need Is Love, Help!, and Hey Jude.
"I like the idea of not letting go of each other," Sir Paul told The Sunday Times.
"You know, when you have somebody you love so much. In many cases it's a relative, and even though they go, you don't want to let go - that's what people say when somebody dies.
"They're in your memory, always in your heart. And, yes, that's certainly true of me and the boys."
Sir Ringo said he would like the group to be remembered "with love".
The drummer added: "How many streams did we do last year? One billion? Three billion? It blows me away. The beat's still going on, you know?"