Musician Sean Lennon has released a song he co-wrote with late Star Wars actress Carrie Fisher.

The track, titled Bird Song, was penned "years ago" by Fisher, Lennon – the son of John Lennon and Yoko Ono – explained in a note on Soundcloud.

He said he felt the need to record the unreleased song following Fisher's death from a heart attack in December, aged 60.

"Carrie and I used to stay up 'til dawn chatting and pontificating about life. They were my best moments," Lennon, 41, wrote.

"Anyway, we wrote a song about staying up too late and hearing the birds sing... The lyrics she wrote with me I think are marvellous."

Willow Smith, the 16-year-old daughter of Hollywood stars Will Smith and Jada Pinkett-Smith, contributed vocals to the unmixed recording.

"It's all so wrong to greet the dawn / The birds sing that awful song, saying 'You don't belong here!'" go Fisher's lyrics, over a tinkling piano and lilting harmonies that are more than a little Beatles-esque.

While best known for her acting and writing career, Fisher periodically showcased her musical chops – often at the insistence of her late mother Debbie Reynolds, who'd long hoped she'd be a singer like her father Eddie Fisher.

In a touching scene from the documentary Bright Lights: Starring Carrie Fisher and Debbie Reynolds, released a week after the pair's death one day apart last Christmas, Reynolds gets emotional while narrating a video of a teen Fisher belting a ballad live onstage.

"Isn't that a great voice? I wish I had it," she says through tears.

Fisher had also joined Lennon and Yoko Ono onstage in Los Angeles in 2010 to perform Ono's 1973 anthem, What A Bastard The World Is.  

"Carrie Fisher was not just a princess. She was a true artist and a good friend to our family," Ono said following Fisher's passing.