When you ask Penny Junor how she thinks her father would have reacted to her latest book, the composure of a professional biographer falters and just for a moment you sense a daughter on the edge of tears. ''Well, I wouldn't have written this story if he were still alive,'' she says. ''It would have been so hurtful to him, so very hurtful.'' But John Junor, for 32 years one of Fleet Street's most influential editors, probably knew that at some point she would unmask him for all those profoundly complex reasons which boil in the bloodstream of filial love.

Lionised by his Sunday Express readers for the brilliant, rampaging populism of his weekly column, JJ, the sage of Auchtermuchty, died in 1997. ''But he was always aware of what he really was. He knew he had killed my mother's love. He knew he could be brutal, and he wouldn't have expected me to go easy on him.'' For 28 years, in fact, Junor's husband, James Leith, had also suffered at his father-in-law's hands. ''He was vile to him. Absolutely vile at every opportunity, but it is not underestimating it to say that he did destroy my mother. He ruined her life utterly.''

With that last reflection the motive for the book becomes clear. Junor has written Home Truths, Life around My Father for her mother, Pam, who found sanctuary from her marital home by living with her daughter's own family for 23 years. Today she is in a nursing home, suffering from dementia; the humiliated victim of a powerful man. The widow whose hidden story, but for Junor's testament, would die with her as if it had not mattered at all.

Yet just as Dominic Carman was savaged by legal and journalistic cronies of his father because his memoir revealed the clandestine cruelty of ''the great George Carman QC'', so Junor is in the firing line of JJ loyalists for exposing his Jekyll and Hyde nature, the raging domestic viciousness behind a monumental career of instinctive genius, wit, and charm. ''And, yes, there was kindness. When he was being nice he was wonderful, the most stimulating companion, and although I was frightened of him I was also hugely proud of him.''

But, even before the book's publication tomorrow, Junor has been accused of defacing her father's memory, of biting the hand that fed her and of seeking to make a fast buck. Gamine and blonde with a willowy grace, she rebuts such charges with quiet insistence. ''I did not set out to betray my father. This book took four hard years to write, so fast bucks have nothing to do with it, and for me it was also a personal voyage of discovery in trying to understand the tragic circumstances of my parents' marriage. Even though he did frighten me I still loved him in that inevitable way that a child loves a parent no matter how tyrannous.''

It seems ridiculous to think of Sir John Junor, the public knight, being a person lacking in self-esteem, but at home he exhibited the bullying rages of someone seething at his own unresolved insecurities. ''He would never admit to any kind of vulnerability. He simply couldn't do that, which is why he had to control everything. He had to be right all the time.''

Junor's only sibling, the journalist Roderick Junor, died of alcoholism three years after her father's own death from a gangrenous bowel. Roderick was 53 and as children he and Penny had been inseparable but in early adulthood he began to side with their father when JJ started to play out his season as a Victorian disciplinarian to protect his daughter's virginity, subjecting boyfriends to vindictive interrogations and rubbishing any prospect of serious relationships.

Her father, she says, was not an alcoholic but his very heavy drinking did exacerbate the devil within him, and there is no doubt that the rogue gene of alcoholism had wrought havoc on generations of Junors: in Glasgow, where JJ was raised, his two brothers and his father had fallen victim to the disease, as had his uncles and a grandfather.

''It is something which terrifies me with my children but they know the calamity that alcoholism has brought us. The two older ones saw the torment it caused my brother, so that memory is a pretty good deterrent, and I drum it into the two younger ones that they must be aware of the dangers. It's a horrifying condition.''

But, with the principal characters now absent from this tale of sorrow, Junor must surely have worried that in writing her book she would be letting the world trample over private ground. ''I have to say that I didn't agonise at all from that point of view. I mean, when I started to go through my father's papers it was like opening Pandora's Box. I didn't know about all his affairs and I felt so guilty about how he had treated people who were once his friends but had then been suddenly dropped like a burning brick.''

Shaped in male chauvinism's inflexible mould, JJ didn't believe in divorce but considered adultery to be a man's rightful game. He was someone who to the end, if challenged, would have professed to love his wife but through some fiendish contortion of the psyche he felt that gave him licence to treat her with contempt. ''And because he was my father I felt in a curious way responsible and I wanted, through this book, to make amends for his behaviour. So I felt that I couldn't not discuss these things because there is no point in embarking on such a pursuit unless you are prepared to tell the truth. After all, as a biographer and a journalist I try to do that with other subjects so how could I censor this one?''

But has the book exorcised the demons? ''To some extent, yes. At one time my mother did plan to divorce my father when she found out about an affair he had with a model, the sister of one of my brother's girlfriends.'' Junor was 12 when JJ told her that he and Pam were breaking up. ''He was crying and so upset that I couldn't bear it and I pleaded with my mother not to do it, but as a result I believed for years after it was my fault that they had remained locked in all that misery.'' Now, from her research, she knows that JJ manipulated her mother into dropping any notion of divorce by emotionally blackmailing her into a belief that if she went ahead, she would be the one wrecking their daughter's happiness.

''She didn't leave him and in missing her moment to go she lost the will to do so ever again. But she was also typical of a generation of women who never had money of their own. She never had a career, she had no parents nearby or anyone to turn to in such trouble because we lived in the country and in those days there were no support systems for women who were mentally abused. And if you endure that sort of battering day after day, if you are criticised for everything, appreciated for nothing, and told in front of others that you are ugly and untalented, stupid and useless, then every ounce of self-confidence is crushed in you and you end up believing the propaganda.''

In preparation for writing the book Junor wrote an article for The Herald requesting anecdotes from anyone who remembered her father from his Glasgow days. The result yielded much valuable information but in the painful trawl through her parents' effects she also unearthed buried treasure: the evidence in love letters that they had been tremendously happy in their first 10 years of marriage. So what could have accounted for the cataclysmic change of emotion? Junor pins much of the blame on the malign influence of that mighty newspaper magnate, Lord Beaverbrook, who was her father's proprietor and mentor at the Sunday Express.

''We lived within a mile of Beaverbrook in what was virtually a tied cottage on his country estate, and as a result my father was at his beck and call all hours of the day and night. But I think power and having to suppress the pain of his stomach ulcers all the time soured my father. Up to that his diaries reveal him as an essentially nice man who was joyful and kind and much more easy-going.''

Yet just why he was so contemptuous of women remains a puzzle. He loved his mother who had her own shop and was by far the strongest member of his family. ''In many ways she was a domineering old bag although he adored her. But he did actually believe that a woman's place was in the home or else in menial work, and his column was often very misogynistic.''

Perhaps the greater mystery, though, is how Junor's husband managed to survive her father's vindictive harangues. The couple had met when she was a student at St Andrews and James was studying drama, later joining Perth Rep. Eventually he forsook acting to run a restaurant and, later still, entered the then uncharted territory of the house husband while Junor concentrated on journalism and royal biographies. Today he works for a company making wooden jigsaws.

''Through all our married life I have been the main breadwinner but that has never caused resentment between us. In fact, James has been such fantastic support I simply couldn't have coped with my father without him. He's very resilient and at the same time sensitive, but I know that when we moved the family and my mother from Surrey to Wiltshire to put 100 miles between us and my father, James and she were much happier characters.''

That was only three years before JJ died, so why did it take Junor so long to make that decision? ''Because I couldn't bear to turn my back on him, but by then I knew we had to do something radical. You see, my father did love me and if you feel that you are loved you can almost take anything else that comes. But it's an incredibly sad story and every time I read the proofs of the book I howled at the heartbreak of it all.''

The only good thing to emerge from the whole exercise, she says, is the discovery of that tender, passionate correspondence between her

parents before mutual loathing overtook them. ''I had always thought that my poor mother had lost out on any married happiness but to know now that my brother and I were conceived in love, not hate, is a great comfort.'' A gift which Penny Junor never imagined would be of such incalculable worth today.

Home Truths, Life around My Father by Penny Junor. Harper Collins, (pounds) 18.99.