A quarter century after John Lennon was gunned down outside his New York apartment building, a struggle remains among his family, friends and fans for control of the former Beatle's legacy.

Defining Lennon has, since his murder on December 8, 1980, become a cottage industry -- pumping out exhibitions, memoirs, album re-issues, documentaries and even a Broadway musical.

The gatekeeper of the Lennon legacy and chief protector of his posthumous image is his widow Yoko Ono, 72, who continues to act as his spokesperson on issues ranging from contemporary music to the war in Iraq.

"I'm sure John would have been terribly upset" about the war, Ono told an interviewer in March 2003.

"And I'm sure that he would have expressed his anger and told them off," she added, referring to US President George W. Bush and British Prime Minister Tony Blair.

Widely believed to have been a major factor behind the Beatles' breakup, Ono has always divided public opinion and her pronouncements on Lennon's behalf have been received with a mixture of respect and irritation.

In the run-up to the 25th anniversary of Lennon's death, other voices have sought to challenge the gospel according to Ono and offer a more earthy image of a cultural hero whose premature death made him an almost saintly figure.

In her memoir "John," published in October, Lennon's first wife, Cynthia Lennon, wrote of the "pain, torment and humiliation" she suffered in her marriage with a man who could be as cruel as he was loveable.

Cynthia, 66, pushed the memoir as "the real story of the real John" -- a message echoed by her and Lennon's son, Julian, in a foreword to the book.

"If there is to be a balanced picture of Dad's life, then Mum's side of the story is long overdue," he wrote.

In a separate statement on the 25th anniversary, Julian Lennon admitted to "very mixed feelings" about his father.

"He was the father I loved who let me down in so many ways," he said.

Ono, meanwhile, marked the anniversary by editing a compilation of glowing tributes from celebrities such as Elton John and Mick Jagger who, according to the book sleeve, "reminisce about Lennon as a visionary and friend, musician and performer, husband and father, activist and jokester."

In her own introduction, Ono said the intervening 25 years had done little to numb the pain caused by Lennon's murder.

"When I'm alone, when the evening light starts to drench the world in pink, in the dark of the night and a dawn, my heart still shakes and will not stop," she wrote.

Ono has authorised a number of retrospective projects over the years, triggering criticism from some quarters that she was turning Lennon into a brand name.

"I don't think I'm merchandising John aggressively at all," Ono once responded in an interview.

"If I didn't do it, then some people would do it and make some kind of cheap version of it or something like that. And it would really destroy John's work. And so I have to protect his work," she said.

Ono has sued former employees for trying to sell stolen Lennon memorabilia, rowed with Paul McCartney over his attempt to reverse the famous Lennon-McCartney songwriting credit, and last month the producers of a Lennon musical in Britain were forced to drastically change the show after she threatened litigation.

An Ono-approved musical about Lennon's life and work played on Broadway earlier this year, but closed after just five weeks following scathing reviews.

Ono has also lobbied hard to keep the man who shot Lennon, Mark Chapman, behind bars, and she recently castigated the NBC television network for choosing to mark the death anniversary by broadcasting an old taped interview with his killer.

Chapman, who was arrested at the scene of the murder, is currently serving a 20 years to life prison sentence.

Now 50, he was denied parole for a third time last year after the parole board cited the "extreme malicious intent" of his crime and said releasing him would undermine respect for the law.

In the interview, recorded more than 10 years ago, Chapman describes how the search for his own personality compelled him to shoot Lennon five times in the back.

"It was like a train, a runaway train, there was no stopping it. No matter -- nothing could have stopped me," he says.