In the first step towards a public inquiry into what could prove the most shocking cover-up in French judicial history, Alain Behr, a lawyer for the parents of one of the missing girls, filed a formal complaint yesterday with the justice ministry alleging obstruction of justice and corruption.

Between 1958 and the early 1990s inquiries into the disappearance of dozens of young women in the region around Auxerre, some 100 miles south-east of Paris, were either mysteriously shelved or so badly handled that only two cases have so far been resolved.

Most of the files have since gone missing.

"It beggars belief that this is mere incompetence. All the evidence suggests a pattern of conspiracy, corruption and protection of people at a very high level," Mr Behr said.

A lawyer acting for the family of Joanna Parrish, a Leeds student found raped and murdered near Auxerre in May 1990, has also asked France's most senior prosecutor to include her among the unsolved cases.

Mr Behr's case involves Isabelle Laville, 17, who disappeared in December 1987. Although her parents long ago gave up hope of seeing their daughter again, they believed the case was at least still open.

But in December last year, the Auxerre prosecutor, Suzanne Le Quéau, learned that the records of most of the criminal investigations shelved in Auxerre between 1958 and 1982 - including 17 cases of missing young women - appeared to have been stolen or destroyed. Horrified, Ms Le Quéau began looking closely at a dozen disturbing post-1982 investigations for which the files still remained. All were inquiries into the fate of missing young women, and all had been started then suddenly, and inexplicably, dropped.

Among them was Ms Laville's case. Just three days after a gendarmerie report in January 1988 stating that she had "in all probability" been abducted and murdered, the investigation had been quietly halted.

Ms Le Quéau's discovery came as a result of a belated and curiously bungled inquiry into the mystery of seven girls with learning disabilities, aged 16 to 22, who disappeared between 1977 and 1979.

At the time, after brief inquiries, all had been listed as runaways. But a little over a year ago, Emile Louis, a bus driver who, despite a record of sex offences, drove the girls to and from their day care centre, was arrested.

He confessed to "fully consensual sex" with each girl, then murdering them and burying their bodies. Only two of the bodies have been found.

Mr Louis, 66, has since retracted his confession, insisting the girls were routinely abused, and finally abducted and killed, by a nebulous ring of men "of some standing, locally and in the region".

It was while preparing the case against Mr Louis that Ms Quéau discovered that six of the seven original files on the missing girls had vanished.

Lawyers for the victims' families, and at least one former official from the Auxerre prosecutor's office, now talk openly of a high-level sex ring that abducted, raped and murdered some 30 young girls in the 1970s and 1980s - and then pulled strings to stifle any subsequent investigations.