The song, which was rewritten specially for the funeral, will keep its original title when it goes on sale next week. All the money raised by the record will be donated to the Princess's Foundation.
It was originally composed as a tribute to Marilyn Monroe but only made it to number five in the charts when it was released in the 1970s. The new version is expected to rocket to the top of music charts all over the world.
John, who was a close friend of Diana's, went straight from his emotional performance at Westminster Abbey to record the single. He was accompanied by Sir George Martin, the man responsible for producing the best of the Beatles songs.
Speaking on the BBC programme Breakfast with Frost yesterday, the singer said: "George suggested I did a piano and a voice live and I did two takes. The second was really, really good. I did some harmonies on it and he added some string quartet and some woodwind. The record will be out on Saturday or Monday. I just thought people might want a reminder of Saturday."
The musician described singing the song as probably the most nerve-racking moment of his life. "When I started singing and playing, I suddenly realised this was it. I was fairly composed all the way through and I sang it well. But at the beginning of the last verse, my voice cracked and I was really choked full of emotion. I had to close my eyes and grit my teeth and get through it.
"It was more emotional when I had finished. It was just a huge relief to have sung it and not get the words wrong."
He said he had used a teleprompter to help him remember the new lyrics. "I just thought, I am not going to mess this up on such a big occasion," he added.