The full extent of Mao's cruelty is revealed in a new biography by 'Wild Swans' author Jung Chang. She talks to Louise East in London.

In 1992, the year after Wild Swans was published, author Jung Chang took a trip to Ireland with her mother and her husband, historian Jon Halliday. In Dublin, Galway and Dundalk (where Halliday grew up), people approached the trio, just wanting to talk about how much Chang's book meant to them.

"Jung's mother doesn't speak English," Halliday says. "But later she said to Jung that it wasn't until she got to Ireland that she realised how big Wild Swans was."

Big is not the word. Wild Swans is the largest-grossing, non-fiction commercial paperback in publishing history, selling 10 million copies in 30 different languages. Thirteen years after its publication, its distinctive jade-green cover still pops up everywhere, from the Waterstone's books-of-the-century list to the Leaving Cert syllabus. The one place you will not see it is in a bookshop in China, where it is still banned.

For many Western readers Chang's history of three generations of Chinese women was an introduction to the bleak history of modern China and to the damage caused by the regime of Mao Zedong. Chang's mother was publicly denounced during the Cultural Revolution, forced to kneel in broken glass before a baying crowd, and Chang herself was exiled to rural China to work as a peasant. So it's somewhat surprising to discover that Chang and Halliday have spent the last 12 years researching and writing a 700-page biography of none other than Chairman Mao.

"Well," says Chang spiritedly. "When I finished Wild Swans, I felt it was a good idea to write about Mao, because he dominated my early life. My family suffered tremendously under him, he brought catastrophe to a quarter of the world's population, and he was as evil as Hitler and Stalin, and yet the world knows astonishingly little about him."

So is Mao: The Unknown Story a way of reclaiming her life from Mao's regime?

"There's nothing personal in it," Chang says firmly. "When I first came to Britain, I used to have nightmares and I couldn't bear thinking or talking about the past. I'd burst into tears at dinner tables. But after writing Wild Swans, I was no longer haunted by the past. So when I came to write about Mao, it was out of fascination for this man. I wanted to know more about him."

From outside, the white house in west London shared by Chang and Halliday offers up few clues about its owners, but inside it's a different story. In the hall there's a framed scroll of delicate Chinese calligraphy praising Wild Swans, in the airy kitchen an oil painting of Jung's mother nestles among rose- coloured stone sculptures, and upstairs a sitting-room with high Georgian windows is dominated by a desk stacked high with Chinese books bristling with neon-coloured Post-it notes.

Halliday makes tea in porcelain cups while Chang has her photograph taken upstairs.

"For a historian, [researching Mao] was fantastically exciting," he says. "Usually you're working over old material, but we found completely new stuff on the whole of his life."

When Chang reappears she gestures to her ornate embroidered Chinese robes and says "sorry about this" as though she is wearing oil-splattered dungarees. At 52, she is strikingly beautiful, with waist-length black hair and a quick laugh. For her, the research with Halliday felt like detective work; she uses the word "satisfying" repeatedly. Certainly, what the pair discovered, through talking to Mao's family, friends and staff (many speaking of what they knew for the first time), as well as Westerners from Henry Kissinger and George Bush senior to Imelda Marcos, is likely to alter radically how Mao is perceived.

Far from organising the Long March, with which he is indelibly associated, Chang and Halliday claim Mao was nearly left behind by colleagues who could not stand him. The famine of 1958-61, which killed some 38 million people, was caused not by mismanagement but by a brutal calculation on Mao's part.

Then there are the extraordinary personal details: Mao did not take a bath for more than 25 years and had servants break in his shoes. Madame Mao, his infamous fourth wife, so hated noise of any kind that she insisted staff in her villa walk with arms aloft and legs apart in case she heard their clothes rustling.

"He was so extremely selfish," says Chang. "When he took power at 55 he reckoned he had 15 years to live, so he wanted to turn China into a military superpower in 15 years. He needed to buy the entire modern arsenal and he didn't have any money to pay for it. All he had to trade was food. He knew his people depended on it for survival, but he didn't care about these tens of millions of deaths. Before his programme started, he said to his inner circle: 'Half of China may well have to die.' Even I didn't realise he was that evil."

Unsurprisingly, the Chinese government told a small group of Mao's inner circle about Chang's approach, and warned them to watch what they said.

"I think it actually helped us because people realised I wasn't going to write the party line, and also that it was going to be an important book," Chang says. "They wanted to trust their memories and material to this big important book."

She laughs at the world's vanity but, moments later, confirms the risks involved in debunking the myth of Mao. When the subject of Chinese archives comes up, Chang looks troubled and pauses.

"The thing is, I would like not to discuss this point because obviously I don't want to," she halts, and begins again. "In China, Mao is still regarded as the great leader who made mistakes so, um, shall we not discuss this? We had access to a lot of personal archives and I don't want there to be even the slightest chance of getting them into trouble."

For herself, she says, she has worries rather than fears about the book's reception in China.

"I think probably I will still be allowed to travel there, because the Chinese government does care about having a good press overseas," she says. "I hope they'll feel that perhaps it's not a good idea to ban me from going to China to see my mother."

She laughs wryly, but later she says quietly: "I now feel more homesick then when I first arrived. London is my home, but if I've been out of China for any length of time, I feel restless. China is under my skin."

In 1978, when Chang first arrived in Britain, fresh from her years as a "barefoot doctor" and electrician ("I got five shocks in the first month . . . I guess if I was absolutely desperate I could change a plug, but I haven't reached that point yet"), she was so keen to forget about China that she often told people she was from Korea.

Yet escaping her home country proved difficult. Staying in Chinese embassy residences, Chang and the other scholarship students were forbidden to go out on their own and were compelled to wear Mao suits in 1970s London.

"When I arrived in Heathrow Airport, I nearly walked into a men's bathroom because I thought the figure on the door wearing trousers must be a woman," she says. "Because in China during the Cultural Revolution we were not allowed to wear skirts. Also, the man walking in front of me had long hair."

Regardless, Chang soon embarked on a series of firsts among her student group: first to sneak into a pub ("the Chinese translation for pub suggested somewhere indecent with nude women gyrating . . . I was rather disappointed"); first to go on a date with a Western man (to Greenwich, where she ate an exotic cheese sandwich - "in my part of China, there was neither cheese nor sandwich"); and eventually, first Chinese student to receive a doctorate from a British university.

Yet all the time she was bashing through barriers, Chang was living with a fear which saw her melt down below car windows when driving past the Chinese embassy. It was only when she met Jon Halliday, in 1984, that her fear subsided.

"This may sound a little corny, but to me he was the one person I felt I could totally, completely trust and things would be all right," she says.

When her mother came to Britain for the first time in 1988, Chang finally heard the truth about her own family's history - and Wild Swans was born. In a life that had already seen an improbable number of changes, Wild Swans brought about yet another reversal of fortune, not least in enabling Chang and Halliday to spend the next decade in pursuit of the truth about Mao.

"Without Wild Swans we would not have been able to write this book," she says. "We were able to devote ourselves to carrying out a huge project which would normally would have required a giant foundation." The end result, Mao: The Unknown Story, may well rewrite history; for Chang, such a result can't come a moment too soon.

"It shows how unacceptable it is to have Mao's portrait in Tiananmen Square," she says. "It's like having Hitler's portrait dominating the centre of Berlin. I would like for China to make a clean break from Mao and his legacy."

Mao: The Unknown Story, by Jung Chang and Jon Halliday, is published by Jonathan Cape, £25

Waterstone's presents Jung Chang and Jon Halliday, in conversation with Eamon Delaney, at the Royal College of Surgeons, St Stephen's Green, Dublin 2, on Thur, Jun 9 at 7pm (tickets: €5; booking: 01-6791415)