Image copyright Getty Images Image caption Shortly before his death in 1980, John Lennon said his wife deserved a writing credit for Imagine

More than 40 years after the song's release, Yoko Ono is receiving a songwriting credit on John Lennon's classic Imagine.

The surprise announcement was made at an annual meeting of the National Music Publishers Association in New York on Wednesday.

They explained the move by playing a clip of Lennon saying Imagine "should be credited as a Lennon-Ono song".

"A lot of it - the lyric and the concept - came from Yoko," he said.

"But those days I was a bit more selfish, a bit more macho, and I sort of omitted to mention her contribution.

"But it was right out of Grapefruit, her book. There's a whole pile of pieces about 'Imagine this' and 'Imagine that.'"

Image copyright Sean Ono Lennon / Instagram Image caption Yoko Ono posed with Sean Lennon, Patti Smith and her daughter backstage at the awards

Ono and her son, Sean Ono Lennon, were at the ceremony to pick up a song of the century award in honour of Imagine, and were not expecting the announcement.

"When they officially acknowledged - through my father's account - that my mother co-wrote Imagine, the song of the century, it may have been the happiest day of mine and [my] mother's life," Lennon told Billboard magazine.

An emotional Patti Smith, accompanied on piano by her daughter Jessie, then performed the song.

The process of adding Ono to Imagine's writing credits is under way but has not yet been completed, said David Israelite, who runs the National Music Publishers Association.

He added that there may be some opposition to the move as, in US law, a song enters the public domain 70 years after the death of its last creator.

Attaching Ono's name to the song will significantly extend the period of time for which it continues to generate income for the rights-holders.

However, as Ono is already a beneficiary of Lennon's estate, the move would not significantly alter the distribution of royalties.

'Petty and absurd'

However, it is interesting to note that Ono previously threatened legal action to stop Sir Paul McCartney changing the songwriting credits for 19 Beatles songs featured on his live album Back In The US (2002).

Classics including Eleanor Rigby, Yesterday and Let It Be were attributed to "Paul McCartney and John Lennon" instead of the traditional Lennon-McCartney.

Image copyright Getty Images Image caption Lennon and McCartney agreed to split writing credits on all their Beatles compositions before they were famous

Ono's lawyer called the move "ridiculous, absurd and petty"; while Ono described it as "totally inappropriate".

"It is very petty," she said. "John and Paul often disagreed on which songs were written by whom.

"If John was here now they could fight it out or maybe they could never agree. The point is he is not."

Similarly, when the Beatles Anthology series was released in 1995, Sir Paul asked the other Beatles and Ono if they would mind the name McCartney appearing before Lennon on Yesterday, a song he wrote alone.

George Harrison and Ringo Starr had no objection but Ono vetoed the idea.

She did not allude to these events at Wednesday night's ceremony, but said a recent illness - she appeared in a wheelchair - had given her a new perspective on Imagine and life in general.

"This is the best time of my life" she added.

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