TO some, John Lennon remains an iconic figure both in the world of music and as an ambassador for global peace. To others, he was simply a member of the Beatles.

But to Julia Baird, his half-sister, he was much more than words can describe. She talks about him in the present tense with unabashed pride - 'genius', 'intellectual' and other glorious descriptions pepper her conversation about him.

"John's a world icon, I'm sure you'll agree. He was one of the most important innovators of the 20th century.

"John's hero was Gandhi. John would have loved nothing more than to be right up there behind him. He was on that track. He would have been campaigning, protesting and marching. He would have had a lot to say about the Iraq war and he would have been a wonderful figurehead.

"People like Bono and Geldof have only been able to take centre stage as they have because John's not here. The two of them together don't make John, in my opinion."

Julia, 59, bears a striking resemblance to her late brother: dark hair and dark eyes, wiry features, today dressed in black from head to toe, wearing a badge with the famous peace emblem on it.

As director of the Cavern Club, where the Beatles first found fame, Julia spends much of her time keeping her brother's memory alive. She danced the night away at the club's recent 50th birthday party.

"I never saw the Beatles at the Cavern," she laments. "Their last date was August 1963, when I was 14 and looked about ten. It must have been a fantastic night."

Yet the Beatles' heyday was very different to the celebrity culture we have today, she points out.

"They weren't out partying in full public glare. They weren't pictured falling drunk in and out of taxis, taking drugs, not caring. It was all behind closed doors. But the fame had to change them because they were always going from the limelight into the dark."

She keeps in touch with Sir Paul McCartney, Cynthia Lennon and her nephew Julian, but hasn't seen John and Yoko Ono's son Sean for 15 years, she says.

Indeed, contact with Yoko has always been minimal. For a long time when John was with Yoko they hardly spoke - when Julia called John, Yoko fielded the calls.

It wasn't until much later that she realised that John had got into drugs. "John was going down that track - I'm not blaming Yoko for that. I don't know what happened."

While John's fame increased, Julia went to live in Belfast with her husband Allen Baird, with whom she has three children. "For me, Ireland was an escape hatch. I was grateful to be away from the furore surrounding John. No one in Ireland knew he was my brother, apart from Allen's family, who very discreetly never mentioned his public antics."

Then, in 1974, a phone call from John re-established contact and from then on he called on a regular basis. She believes that he wanted to return to Liverpool when he was shot five times by crazed fan Mark Chapman as he returned to the apartment he shared with Yoko in New York's Dakota Building, on December 8, 1980.

Now Julia has written Imagine This, a memoir about their early life. She has written it to rebuff stories that their mother was an neglectful woman because she let five-year-old John go and live with his Aunt Mimi.

Aunt Mimi was a formidable, frightening and domineering woman, by all accounts. Yet despite the fact that Mimi took John away after claiming to social services that his mother wasn't fit to look after him, Julia says she bears no malice or hatred towards her.

"I felt anger. I was agitated on account of her agitation. I tried to get Aunt Mimi to talk, but didn't dare. Even on her death bed we were all intimidated."

Theirs was a complex family set-up. John was born to Julia and Alf Lennon, whose marriage collapsed because Alf was so often away at sea. When John was four, his mother had an affair with a young Welsh soldier and became pregnant. She was condemned by her family and told she couldn't keep the child, a girl called Victoria, whom she was pressured into putting up for adoption.

Later, she started a relationship with another man, Bobby, who moved in with her and assumed the father's role. Together they had two daughters, Julia and Jackie, and he treated John like his own.

But the rest of the family vehemently disapproved of the fact that Julia was still married and living with a man with whom she had two illegitimate children. This was why Aunt Mimi stepped in.

The issue of the secretly adopted baby was kept from Julia and her siblings until adulthood. She says the trauma of being taken from his mother, who he loved dearly, scarred John for life. But as he grew older, John saw more of his family, popping in whenever he could.

Then a second tragedy struck. Just as he had got to know his mother again, she was run over by an off-duty police officer with no driving licence or insurance. She died instantly.

John said later: "I lost my mother twice. Once as a child of five and then again at 17. It made me very, very bitter inside. I had just begun to establish a relationship with her when she was killed. We'd caught up on so much in just a few years."

Julia says she has written the book not to fuel the memory of John, but to exonerate her mother, who has been maligned over the years for 'giving away her son'. Julia maintains her mother was forced by her father and pressure from the rest of the family to give John up.

Julia's 13-year marriage broke up the same year that John died, but she won't talk about it. She now lives with her partner, Roger Keys, in Chester, where she was a special needs teacher for 15 years. She's also a fully trained psychotherapist - "that way I don't need counselling".

Like her brother, she has supported good causes, volunteering in India with Save the Children, and retaining strong links with communities in the Himalayas.

Looking back, Julia says she wishes John hadn't become famous, because then he would still be alive today. But she feels the need to remain close to her roots.

"I still feel John's presence in Liverpool, because it is still there. I need to have the beautiful Welsh mountains and the sea within easy reach, where I can touch my reality. Liverpool has become my umbilical cord."

And she hopes the book will explain her mother's actions to those who still don't understand. "My mother was a beautiful, vibrant and loving woman. And she was wronged. 'They've taken my son. They've taken him from me. They won't let me have him. I have to get him back.' My mother's heartbroken words.

"She has him now."

* Imagine This by Julia Baird (Hodder & Stoughton, £18.99).