Marked “secret” and signed by Heinrich Himmler, the bureaucratic language of document 664-PS dated 10 March 1944 masks the genocidal reality of its real meaning.

It reads that as far as Jews and Gypsies are concerned, “the accomplished evacuation and isolation of these groups by the chief of the security police and the SD [SS intelligence]” had made previous prohibitions against them now meaningless.

It’s a chilling directive, and one put before the Nuremberg war crimes trials. A copy of the translated document is now to go on public display, along with eyewitness accounts, many made public for the first time, testifying to the Nazi genocide against the Roma and Sinti in an exhibition focusing on “forgotten victims”.

Facebook Twitter Pinterest Unpublished accounts of survivors of the Roma genocide collected in the 1950s by the Wiener Library. Photograph: Wiener Holocaust Library Collections

In essence, the directive means that all Jews and Gypsies, at least according to Himmler, had at this point been removed from the Reich. “They have been forced out, or killed. The whole of the Reich has been cleansed in this murderous way,” said Dr Barbara Warnock, curator of the exhibition at the Wiener Holocaust Library in London.

“The language is bureaucratic and administrative, but in fact it is talking about genocide. It’s just so chilling to read. It is also very striking at this stage, quite late on, his lumping together of Jews and Gypsies”.

Up to 500,000 Roma, of eastern and south-east Europe, and Sinti, of western and central Europe, were murdered by the Nazis and their collaborators. In Romani it is known as the Porajmos, or the devouring.

They are the “forgotten victims”. Their fight for official recognition of this genocide, and for restitution, lasted decades. They were not the focus of the Nuremberg war trials. Germany initially refused to recognise it as genocide, claiming Roma and Sinti were imprisoned and murdered in camps such as Auschwitz for asocial and criminal behaviour, not for racial reasons.

The evidence to the contrary is in the testimonies, forming part of the exhibition, of survivors subjected to horrific treatment and, often, enforced medical experiments.

Facebook Twitter Pinterest Dr Robert Ritter taking blood samples in 1938 as part of his pseudo-scientific racially based ‘research’. Photograph: Galerie Bilderwelt/Getty Images

One account, by an Austrian Roma woman, Hermine Horvath, is, unusually, explicit about the sexual abuse suffered. An SS man, on whose farm she was a slave labourer, threatened her. “I noticed very quickly that this [local SS leader] did not worry at all about the racial problem when it came to a young Gypsy girl,” her witness account reads. “He started to come after me. One day, he was suddenly standing in front of me with a drawn pistol.”

Horvath told the SS leader’s wife and was protected. Others were not so fortunate. Following her deportation to Auschwitz-Birkenau in 1943, Horvath recounted: “It was especially terrible for us that we had the SS men staring at us in our undressed state all the time. One SS squad leader in Block 8 took women whenever and wherever it suited him.” She tells of the rape and murder of two Gypsy children by an SS officer in Auschwitz.

Horvath died shortly after giving her testimony, aged 33. She had suffered as a slave labourer and was the victim of forced experimentation. Her sister died of starvation in Ravensbrück concentration camp.

“Her account is unusual because there was a reluctance to speak about sexual violence, possibly to protect their families, possibly, and unfortunately, because of a sense of shame themselves,” said Warnock.

Facebook Twitter Pinterest Margarete Kraus (left) and Vinzenz Rose’s certificate of incarceration, showing that he ‘geflüchtet’ (escaped). Photograph: Wiener Holocaust Library Collections

Possibly, also, because they were never asked. “In terms of investigations into the crimes of the Nazis, it was not what the attention was focused on,” Warnock added.

The records of Margarete Kraus, a Czech Roma sent to Auschwitz aged 13, detail the appalling conditions of the Gypsy family camp there, the Zigeunefamilienlager. She was transferred to Ravensbrück and survived, but the Gypsy camp would be shut down on 2 August 1944, when about 3,000 remaining inmates were sent en masse to the gas chamber. Today, 2 August is the official commemoration day for the Romani genocide.

Other documents relate to Vincenz Rose, a German Sinti, who endured medical experiments but managed an audacious escape from Auschwitz. He and his brother Oskar later hired a private detective to track down Dr Robert Ritter, a Nazi “racial scientist” and architect of the experiments Roma and Sinti were subjected to. Investigations into Ritter were later discontinued.

Warnock hopes the free exhibition Forgotten Victims will highlight the nature and extent of the persecution and genocide carried out against them across Europe through the stories and photographs of those who experienced it.

“They are still the forgotten victims. Many people are not really aware of the extent to which Roma and Sinti were targeted,” said Warnock. “Across Europe, Roma and Sinti face such high levels of discrimination, and frequently poverty, it continues to make it harder for these events to be remembered.”

• Forgotten Victims: The Nazi Genocide Against the Roma and Sinti is at the Wiener Holocaust Library, London, 30 October to 11 March