Timothy Garton Ash tells Harriet Swain what it was like to be spied on by friends and colleagues working for the East German secret police

Our first glimpse of Timothy Garton Ash in his new book, The File: A Personal History, reveals him lying on a bed in a Berlin tenement, preparing to make love. Now, clothed in foreign correspondent garb of pale suit and open-necked shirt, he offers tea - Earl Grey (no milk or sugar) or herbal. His gingery beard and book-lined room in St Anthony's College, Oxford, are neat.

He hesitated for a long time, he says, before writing the book, in which he investigates the file kept on him by the East German secret police, the Stasi, before the fall of the Berlin Wall. "The thing can seem so self-indulgent," he explains. "One hears such strange reactions from other people. They say things like 'you're so lucky to have a file' or 'it's such a privilege'. One person said: 'Oh, I think my file's in Russia'." He calls the phenomenon "file envy" and reminds me, a couple of times, that his code name was "Romeo", which only added to the difficulty. "One has to be a little bit leery of being sure it isn't a self-indulgence or boasting," he says.

The fear of self-indulgence is surprising because the book is actually deeply disturbing. Garton Ash was one of millions of people whose movements were observed and recorded with varying degrees of intensity by the Stasi in the former German Democratic Republic. Like more than a million others, he chose, after 1992, to find out what had been written about him. To do this, he was meticulously aided by the "Gauk" authority set up by the German government to store, label and reveal the covert work of its Communist predecessor in the East. Using the historian's technique of seeking and weighing evidence, he compared what he found in the file with his own diary entries of the time and with contemporary interviews with some of the people who appear in it. "It would have been easy to do it in a novelistic way," he says. "But it's history and everything in the book is true."

³ÉÈËVRÊÓƵ

The bed scene describes what went through his mind when he first opened his file. He feared his then girlfriend might have been an informer. She was not. But five other people were and he sets out to track down each one, does his own snooping into their backgrounds and tries to surmise whether, given similar circumstances, he or anyone else could have become informers too.

His research reveals a sad list of ordinary people, some likeable, some not, enticed into surveillance work through fear, conviction or the promise of travel and glamour. The first was a "smart Alec" academic at the Humboldt Institute, where Garton Ash had studied. The second was "Michaela", whose husband had been married to Kim Philby's widow and who spied through belief in the system, her desire to use the position to visit America and fear. Then came a rather pathetic English lecturer, anxious to travel and blackmailed over drinking and homosexual advances to students, followed by an Englishman, Smith, who feared having to leave his German wife and child behind the Berlin wall unless he collaborated. Finally, there was an elderly Jewish woman, who had spent more than ten years in Soviet camps and whose first child had fled to the West. What they found was strikingly banal. One described a meeting between the young Garton Ash and a woman in East Berlin, recording what they were wearing, the cafes they visited, how they greeted each other. Another described his room in the city. Others record that he was rude during a visit to a German scholar's family in failing to notice hints that his hosts wanted the visit to end and that, on a visit to another family, he suspiciously failed to recognise someone's daughter.

³ÉÈËVRÊÓƵ

His name is spelt slightly differently throughout the file and dates do not always tally with those in his diary of the time. But Garton Ash says that the argument of many Germans that "the files lie" is not exactly true. "My basic finding is that they were pretty damn accurate," he says. "The interpretation is paranoid. But everything in between is chillingly accurate."

His story is disturbing on a human level because it reveals such unpalatable aspects of relationships, weakness and memory. It is disturbing as a historical work because it raises difficult questions about the validity of historical sources and interpretation. And it is disturbing on both these levels because it shows the continually shifting basis of truth. But most troubling is that it is all so recent.

Garton Ash is a lot younger than his precise intonation would have you believe. Born 42 years ago and educated at Sherborne and Exeter College, Oxford, he had the most stable and happy childhood anyone could wish for, he says, explaining an important difference between his own background and that of many of the people who informed against him. Even then, he flirted for a short time with joining the British secret service, struck with the romance of the idea, a hangover from the character-building experience of being sent to boarding school aged eight, "which required you to learn, very young, both self-reliance and the habits of secrecy".

He went to Berlin as a PhD student in 1978 to write a thesis on Berlin under the Third Reich but while there became fascinated by what was happening in East Germany and Poland. He abandoned his thesis and concentrated on writing about the GDR. Then came a book on the Polish Solidarity movement. Perhaps his best-known book is We the People, which describes the revolutions of 1989 in Warsaw, Budapest, Berlin and Prague from first-hand experience. He is, he stresses, a contemporary historian. "A lot of my work is on the frontier between East and West, on the frontier between scholarship and journalism," he says. "They are interesting and instructive places, if often uncomfortable."

By describing the process of tracking down his file, he wanted to bring home to people what it was actually like living in a police state - an experience not properly understood even by those in West Germany. "What I have tried to do is explain," he says. "It is absurd that an Englishman has had to explain this sort of thing to a West German."

To do this, his "personal history" employed many journalistic tools. He charmed and blagued his way into interviews with past informers like any good tabloid hack, while being scrupulous in trying to understand their motives. "Good journalism and good contemporary history have a lot in common and have a lot to learn from the other," he says. "Historians are far too reluctant to write about the recent past. Journalists spend too much time working out what's going to happen tomorrow. A lot of what I have tried to do throughout my work has been the history of the present - writing about very recent events from a historical perspective."

³ÉÈËVRÊÓƵ

Researching his file made him rethink his ideas about history. "One of the lessons I took away is that we need to take more account of the extraordinary creativity of memory. I came away thinking if I couldn't even reconstruct what I was like, how can I try to write someone else's history? What you don't come away with is postmodernism. You don't come away with the anything-goes idea because this is too important a business. Some people's lives were ruined as a result of these files. You come away with a healthy respect for facts but a healthy scepticism about interpretation and particularly about retrospective interpretation."

Behind all his work - historical and journalistic - is a long-held passion for Central Europe, derived from no family connection but from Thomas Mann. "I was fascinated by German literature," he says. People who have read his work, or who meet him in the course of his travels, find it hard to believe he has no German blood. When he first travelled to Central Europe, he felt an immediate affinity. "It was, if not love at first sight, fascination at first sight, and that continues." What struck him was "the proximity of great beauty and great evil, which is the great German puzzle".

³ÉÈËVRÊÓƵ

James Fenton, his then flatmate and now professor of poetry at Oxford, remembers him as "very smitten with Berlin". "He is one of those English people who find a new role in a different language and find living abroad and inventing a personality for himself liberating. When he speaks German he becomes a German. He's very British when he's British and very German when he's German."

Fenton recalls that while Garton Ash was "never a dry academic" and always maintained an impressive contacts book, his first priority was research. His contacts were all people who could talk to him about what it was like living in Berlin in the 1930s. It was only later that he began to concentrate on journalism.

He started at the Spectator, where he was the foreign editor, also writing commentaries on Eastern Europe for The Independent. In 1980 he was at the Lenin shipyard in Gdansk, sitting with striking dock workers. He spent the year in Poland, as a journalist and also as the supporter of Solidarity. It ended when he was given 24 hours to leave the country.

Since then he has been to Czechoslovakia, Hungary, Albania, Bosnia, to witness key moments in their histories. He was one of those at the infamous Chequers meeting in March 1990, when Margaret Thatcher expressed her concerns at German reunification and the "unreliability" of the German character. He is also married to an East European, Danuta, whom he met in West Berlin but who previously lived in Cracow. The couple have settled in Oxford, where Garton Ash has been a fellow of St Anthony's College since 1990 and their two sons attend local schools.

He ends his book with thoughts of these sons, sparked, he says, by the realisation of how important fathers are. By finding out about the people who had informed on him, he found most of them had lost a father at a formative period in their lives. "What I take away from this experience is the picture of those lost children the Stasi gathered in - this terrible question of how you can arm your children so they aren't caught up by whatever the temptation is. In a dictatorship, it's working for the Stasi, in free societies it's something else."

For many people, who found their informers were close friends, sons-in-law, even husbands, discovering their files has been a deeply shocking experience. But Garton Ash believes in most cases it has been a relief. He describes his own as "a gift". "In a way, everyone should have one," he says. "It's interesting, particularly in the middle of one's life to stop and see yourself 20 years ago. You just don't want to have to have the Stasi to make that happen."

³ÉÈËVRÊÓƵ

The File: A Personal History is published by HarperCollins, price Pounds 12.99.