Jewish leaders in the UK have demanded an explanation after French authorities in Strasbourg admitted that the remains of dozens Jews killed in the Holocaust then used in Nazi experiments had been retained.

A medical institute was forced to come clean about the surprise retention of body parts in jars and containers in the French city for 70 years, during which time they were thought to have been buried in a mass grave in 1946.

“This discovery raises serious questions about who knew about these remains and why they did not reveal this sooner,” said a Board of Deputies spokesman.

“We would expect a prompt and full investigation in to these matters, and the remains of these victims to be accorded a respectful burial in accordance with Jewish law as soon as possible.”

The Jews, thought to number 86, were brought from Auschwitz to KL-Natzweiler and killed in gas chambers over one week in 1943. This was for the express purpose of providing body parts to an anatomical laboratory, where August Hirt, an SS captain and chairman of the Reich University in the city, built up a disturbing collection.

Hirt committed suicide before he could be tried for war crimes, and his display, which was supposedly designed to show the “racial inferiority” of Jews, was discovered after the Allied liberation of Alsace in 1944.

In the months that followed, Nazi record-keeping meant that each of the 86 victims could be traced and identified, but it is unclear why the remains were kept.