Published: 3:12PM BST 10 Oct 2009

The voice on the airwaves was calm and clear. "I am Sofia Davidovna Miliband," the surprise caller to a Moscow radio station declared. "I am your relative. I am the only one left." Sat listening in the studio, British minister Ed Miliband, who was giving an interview during an official trip to Russia last Tuesday, almost fell off his chair. He and his brother David, the Foreign Secretary, had a vague inkling of a long-lost distant cousin, but believed she was dead. "I want to tell you about your relatives," the voice continued, adding somewhat cryptically. "I was dead, but then they resuscitated me."

Perhaps not surprisingly. the talk show host then cut the call off, assuming it to be a crank. But a little later, Mr Miliband, who was in Moscow on a visit as climate change secretary, would be thanking the radio station for making "great things" happen. A chance call, it turned out, had achieved what the rise of Nazi Germany and the descent of the Iron Curtain had thwarted - the reunification of the Miliband clan. By the end of the day, Mr Miliband was in his cousin Sofia's Moscow living room, quaffing champagne and listening to an extraordinary life story that navigates one of the most tumultuous and terrifying periods in modern history.

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In an exclusive interview with the Sunday Telegraph, Sofia Davidovna, who recently suffered a near-fatal heart seizure, spoke of the joy of finding a long-lost relative and recounted the terrible fate that befell many of the brothers' extended family. Despite facing systematic anti-Semitism in the Soviet Union herself, it turns out that, against the odds, she was also a high-flyer. Wheelchair-bound, diabetic and with a weak heart, Sofia Davidovna has led an extraordinary life of her own, including a rare meeting with Soviet dictator Josef Stalin, a stint as a secret Soviet code-breaker during the Second World War, and a prestigious career as a top Russian academic with more than 300 published works to her name.

Now 87, her lively eyes sparkle as she speaks and she uses her thin arms to gesticulate excitedly when she is talking. She is delighted by the surprise reunion that comes, by her own admission, in the twilight of her own life.

"It took my breath away," she says with a smile. "My cleaner was listening to the radio and suddenly she told me my name was mentioned. I called up spontaneously and could not believe it when I got through at the first attempt." She says she had never heard of Ed Miliband. "I knew about David Miliband, of course, and thought we were relatives, but this was the first time I had heard about Ed."

In the academic institute where she worked for much of her life, colleagues say they remember her claiming she was related to Britain's young Foreign Secretary. But until last week, those claims merely prompted laughter. Sofia says she was deeply moved by Ed Miliband's impromptu visit to her book-filled flat, a visit that made him two hours late for an embassy reception where guests were waiting for him. "Ed felt right at home," she recalled. "He is very easy going and we drank champagne and luckily there were cakes and biscuits on the table. It was a proper family reunion. He has the family smile."

Indeed, he was so fascinated by his long-lost elderly relative that he paid a second visit, studying black and white photographs of relatives he never knew he had. It turns out that the Milibands' great-great grandfather Abraham was the brother of Sofia's grandfather Osip. For Sofia, the reunion was timely. Earlier this year, her heart stopped for four minutes and doctors had to revive her. "I almost died," she says. "But I was not afraid. The colours were beautiful and there was a big white cloud."

Neither brother had known for sure that they had a surviving direct relative. Their father, Ralph, a prominent Marxist academic, had snatched a brief meeting with Sofia in the early 1960s in Moscow at the height of the Cold War. But Miliband senior had died in 1994, taking much of the family's tangled Polish-Jewish genealogical secrets with him.

A childless widow, Sofia says her resuscitation made her ponder her life. "When I woke up I realised that I was the last of the Milibands in Russia. That after I die, the surname will disappear. Now at least I have found some more Milibands."

Born in 1922 in Moscow in the early days of Soviet power, Sofia Davidovna was the sole daughter of David and Pearl Miliband. Her roots, like those of the Miliband brothers, are in Warsaw's nineteenth century Jewish community. Her Polish-Jewish father, David Miliband, moved from Warsaw to Moscow in the early part of the twentieth century. That was when the different parts of the family went their separate ways. Instead of going east, the Miliband brothers' grandfather, Samuel, moved to Belgium before eventually settling in the UK.

In 1921, Sofia's parents married in Moscow. Her mother was an Estonian Jew. Sofia was born the following year, just two years before Vladimir Lenin's death. The family was poor. "Life was hard," she remembers. "I remember getting my first dress made from new material rather than second-hand cloth." Her father owned just one suit and worked in a factory, while her mother was a housewife. Sofia, though, was bright. Like other children at the time, she joined various Communist youth movements, the highly politicised equivalents of the Boy Scouts and Girl Guides. She also, by her own admission, hero-worshipped Stalin and went to school in central Moscow, rubbing shoulders with the children of the Soviet elite. She studied at the same university as Svetlana Alliluyeva, Stalin's daughter, and remembers briefly meeting Stalin at one of his secret dachas in then the Soviet republic of Georgia. "I could not breathe or eat anything afterwards I was so excited to see Stalin," she remembers.

Despite her Jewishness, a real disadvantage in anti-Semitic Soviet Russia, her fortunes rose quickly. In 1937, she says she talked her way into an elite diplomatic academy supposed to be only for boys. Her perfect knowledge of German, a language she learned from her mother, swung it for her. Stalin's great terror somehow passed her family by, but she says she still remembers the oppressive feeling of the time. "They were terrible times. We lived in fear," she says, her eyes glassy with emotion. "I remember father keeping a packed suitcase near the front door in case he was arrested. It contained underwear, a warm jacket, soup and tooth paste."

During the Second World War, a period she is reluctant to discuss, she says she helped decipher secret German military communications but took an oath never to disclose the details. She recalls being given several days to translate sackfuls of captured German correspondence, the sorrow she felt in reading despairing German soldiers' letters home, and having to relocate at short notice as the Nazis approached the Russian capital. In the meantime, her relatives in Poland -- relatives of the Miliband brothers too -- were suffering. The Nazis killed one of her uncles, a fireman, in 1941. Facing the prospect of one-way tickets to the Nazi concentration camps, two other uncles killed themselves.

Yet for someone who has witnessed so much suffering, Sofia Davidovna remains cheerful. Having met Ed Miliband, she is now looking forward to meeting his brother David, who is due in Russia next month on a diplomatic mission. "He is going to drop in," she says. "All this attention has made a film star of me."