The 66-year-old former Beatle feels so strongly about the US heiress he has put his emotions into music for Nancy, 48.
A source said: "The song Paul has written a song for Nancy looks set to be on his new album."
"Paul has long been inspired by the women in his life and Nancy is no different."
Earlier this year, it was revealed Paul had collaborated on a song about his ex-wife Heather Mills - from whom he suffered a bitter divorce in 2007 – with world music artist Nitin Sawhney.
A source said of the track: "Paul has done a track about how he feels about Heather because no one has heard his side of it at all."
"It's a very emotional and very powerful song."
Paul has also previously composed songs for his first wife Linda, who passed away after a battle with breast cancer in 1998, including "Maybe I'm Amazed" in 1970.
A number of Paul's songs for The Beatles, including "And I Love Her" and "Here, There and Everywhere" were said to be inspired by his ex-fiancee, Jane Asher.
Meanwhile, Paul has been spotted kissing Nancy on his boat named after Linda.
The couple were sailing at The Hamptons, on New York's Long Island, while on holiday with Paul's four-year-old daughter, Beatrice, with Heather.
Nancy is presently in the process of divorcing her lawyer husband of 23 years, Bruce Blakeman.
