Eva Schloss will never forget her 15th birthday. On May 11, 1944, she and her mother had just moved to a new hiding place in the Netherlands - their seventh. But the Dutch nurse who had led them there then betrayed them. She was a Nazi double agent; they saw her later that day with her own keys to the Gestapo HQ in Amsterdam where they were taken for interrogation.

“I was in shock when the Nazis arrested us,” said Eva. “I didn't cry at first. My mother yelled that I was not Jewish, saying she'd had an affair with a German. I did have blonde hair. But it didn't help.

“Then I was beaten. I was asked repeatedly for the names of Dutch Resistance people who had hidden us. Luckily I never knew their real names. But the Nazis threatened that if I did not tell them they would kill my brother, Heinz, who was not with us in the latest hiding place but whose cries I had heard in the police station.”

Throughout that whole day she was kept in a cell without food or water, having to listen through the walls to the terrified screams of others being tortured. The next day she and her family - her parents, Fritzi and Erich Geiringer, and Heinz - were reunited on a packed train to Westerbork, a Dutch holding camp. Thus began a year-long journey of such unimaginable horror and cruelty that Eva was able to talk about it only after almost 40 years. Few of those on that train were to survive the ordeal. But that day they tried to keep each other's spirits up with hopes of an imminent Allied invasion.

They were soon moved from Westerbork to Auschwitz-Birkenau; here the men were separated from the women. As they parted Eva's father reminded her of the need always to wash her hands properly so that she didn't catch any diseases. “Then he apologised that he could not help me any more.” She managed to see him, secretly, a couple of times in the camps but she never saw her brother again.

Now approaching 80, Eva Schloss, Anne Frank's posthumous stepsister, devotes her life to educating people around the world about the importance of tolerance and respect for difference. As she tells me her story on a cold winter's day I am stupefied at her lack of bitterness.

“Otto Frank, Anne's father, helped me a lot after the war,” she says simply. “He married my mother in 1953 and they had 27 very happy years together. He always said he had no hatred for the Germans. After all, he was a German himself and proud of it.”

Eva was born in Vienna in 1929 into an assimilated, middle-class family. Her father was a shoe maker, her mother and brother played piano duets. But when Hitler invaded Austria in March 1938, life for Jews became impossible. The Geiringers emigrated hurriedly, first to Belgium, then the Netherlands. They rented a flat in a square called the Merwedeplein, and it was here that Eva met the Frank family - Otto, Edith, Margot and Anne - whose apartment was directly opposite. “We children all played together outside - skipping, hopscotch and marbles - and one day a girl ran over to me and introduced herself. ‘I'm Anne and my family comes from Germany,' she said.”

Eva was one month older than Anne. “But Anne was much more mature and grown-up than me,” Eva insists. “She attended the local Montessori school and was an academic year ahead of me. I went to an ordinary local school.” Eva recalls the kindness of Anne's parents in those days, especially how Mrs Frank would often prepare lemonade for the children.

The Geiringers were betrayed and deported to Auschwitz-Birkenau some months before the Franks were eventually discovered. Having survived the first “selection” on arrival at the camp, Eva remembers being humiliatingly showered and shaved and the intense embarrassment of having to submit to a stranger scraping a razor all over her body, then using blunt scissors on her head, before being made to parade naked in front of SS officers and having the obligatory number tattooed on her arm. Worse was to follow over the next months, including a bout of typhus with high fever and delirium that nearly killed her. Having pulled through, she found the courage that never left her. “I determined to survive the war, no matter what they did to me.”

Eva and her mother learnt camp rules: to hold on to a mug and a utensil even in the shower; how to find occasional carrot tops in the rubbish heap behind the kitchen. Eva's good fortune in being given work briefly in “Canada” - the prisoners' name for an elite work unit where the belongings of incoming prisoners were sorted - no doubt also helped her and her mother to survive. But this was followed by much heavier work. Half starved, she sat all day in an icy room with frozen feet and hands, tearing tough fabric into strips and plaiting it. “If you did not achieve your target each day you were removed and gassed to death.”

In January 1945 the Russian army arrived and the Germans made a hasty retreat, forcing those who could to march towards Mauthausen. It was here, according to the Red Cross, that Erich and Heinz died. Meanwhile, Eva and her mother were evacuated by the Russians towards Odessa in cattle trucks.

The Russian soldiers were, according to Eva, kind, with a sense of humour. The survivors were treated humanely and even had a little stove. But the journey was still traumatic and for one terrible week Eva thought that she would never see her mother again. Everyone had to get off the train to relieve themselves and after one such stop the train started without Fritzi. Eventually she discovered where it was going, and mother and daughter were reunited several days later.

By July 1945 Eva and Fritzi were back in their old flat at Merwedeplein. Here they met up again with Otto Frank, who by now knew that his wife and two daughters had been killed. “He was a broken man. He had lost everything. But he came to see us a few days later with Anne's diary, which his friend, Miep Gies, had found and kept for him.”

Otto Frank, helped in his work by Eva's mother, his second wife, devoted the rest of his life to spreading Anne's message around the world. “I still believe that deep down human beings are good at heart,” she had written.

Eva herself was rootless. “Before the war I thought of being a tulip grower. Now it was decided for me that I would be a photographer and was sent to England to study.” It was here that she met her husband, Zvi Schloss, whose family had also been German refugees. The couple remained in England and had three children, Caroline, Jacqueline and Sylvia, all of whom looked on Otto Frank as their grandfather. But Eva had grown into a painfully shy young woman.

“I had been outgoing before the war, but all my confidence had been knocked out of me,” she says. “I was just 16 when the war ended and I wanted everyone to know what I had suffered and feel sorry for me. But no one wanted to hear. So, like all survivors, I just buried my thoughts. I couldn't sleep properly and I had nightmares. There was no counselling or therapy available.”

Then, in 1986, a travelling exhibition came to London called Anne Frank in the World, 1929-45. “My mother and I were invited to the opening and the then GLC head, Ken Livingstone, insisted we join him at the top table. As he ended his speech he turned to me, saying, ‘and now Eva wants to say a few words.' At first I was paralysed and wanted to crawl under the table. I had never spoken of my experiences. But then suddenly everything came flooding out.”

After that, lots of people invited Eva to speak. At first Zvi had to write her speeches. But then, with help from a friend, Evelyn Kent, she decided to write a book about her life, Eva's Story, which was first published in 1988. “And the extraordinary thing was, I knew all the little details that I had kept inside me for so long. Once I let go I could not recall them any more. I was so pleased to get rid of it all that I find now I have to go to my own book to look them up.”

In 1995 the American playwright James Still wrote And Then They Came for Me: Remembering the World of Anne Frank, an educational multimedia play that weaves video of interviews of Holocaust survivors - including Eva Schloss - with live actors recreating scenes from the Second World War. The play is regularly performed all over the world to schools and Eva often talks to audiences afterwards. “If propaganda and advertising can influence young people to do horrible things, we must also educate them to do good,” she says.

The Promise: The Moving Story of a Family in the Holocaust, by Eva Schloss, is published by Puffin Books

Times recommends

• Ask me about sex, not pay
• Culture clubbing
• Honor Blackman: Words from the wise