The files, stacked on desks in a former garage for SS guards at the Ravensbrück women's concentration camp museum in Germany, provide evidence about one of the most sordid but least known aspects of Nazi rule. They recall how hundreds of women, written of as "antisocial elements" by the Hitler regime, were arrested, dispatched to camps and forced to work as prostitutes for slave labourers during the Third Reich.

The plight of the hundreds of women who suffered this fate is the subject of an exhibition which opened last week at the former Ravensbrück camp's museum, north of Berlin. It breaks a taboo on an issue which has remained a virtual secret since the end of the Second World War.

"Hardly any other part of concentration camp history has been so repressed and so tainted with prejudice and distortion," said Insa Eschebach, the museum's director. "The women prisoners who were forced to work as prostitutes remained silent after 1945. Hardly any applied for financial compensation because talking about their experiences was too degrading for them."

Yet with the help of testimonies by former Ravensbrück prisoners, excerpts from Nazi SS files and accounts by camp guards, the exhibition manages to capture the horror and degradation suffered by the Third Reich's sex slaves.

Antonia Bruhn, a former inmate at Ravensbrück, where most of the prostitutes were recruited, recalls in a video interview how the women were lured with promises that they would be set free after six months, fed fresh food and vitamins and tanned with sun lamps to improve their looks. Unlike other women prisoners they were allowed to keep their hair. "After they were primped up, they were all tried out by a group of SS guards in the camp operating theatre. Then they were sent off to the concentration camps to work. Of course none of them were set free as the SS had promised."

The women were forced to work at 10 camps, including Auschwitz, from 1942 until 1945. In special brothels equipped with tiny "copulation cells" the women were obliged to receive eight men a day and up to 40 each at weekends. Sex was only permitted lying down in 20-minute sessions and was controlled by SS guards who watched through spy holes.

Irma Trksak, another inmate, recalled the victims returning from a six-month stint at one camp. "They came back as wrecks. God knows how many men they had had to sleep with. They were ruined, sick and many died afterwards," she said.

The idea was conceived by Heinrich Himmler, the Nazi SS chief, as an incentive for slave labourers. But it was also designed to combat the spread of homosexuality in all-male labour camps. German prisoners were the chief beneficiaries.

The exhibition reveals how the SS delighted in making lesbians work as prostitutes in an attempt to "convert" them. Homosexuals were also forcibly sent to have sex with prostitutes.

On their return many of the prostitutes were subjected to medical experiments and several died as a result.