A doctor who told a patient he was giving her fertility booster when he inseminated her with strangers' sperm is being sued by the horrified mum-of-three and her partner.

The couple, from Vorarlberg, Austria, launched the case when they discovered that all three of their children had different fathers.

It is understood that the pair, not named in court documents, only found out the truth when one of their daughters investigated her own DNA while she trained to become a doctor.

The couple, originally from Turkey, became patients of the doctor's clinic in Styria in the 1990s when they had problems conceiving.

They claim that without telling the wife what was being done to her, the medic had used sperm from other patients on her during 14 different treatment sessions.

The mother simply believed she was being given fertility-boosting drug therapy and expected to conceive naturally with her husband.

One of the couple's daughters used her medical training to investigate her own DNA when she noticed physical differences between herself, her sisters and their parents.

Her research, the family's lawyer Lisa Holzmann says, shows the 'dad' who brought her up could not have been her biological father.

And further checks showed her two sisters also had different biological dads.

When the daughter found her natural father it emerged he had been a patient at the clinic for a hormone test and his sperm had been harvested and used without consent.

The family is still trying to track down the fathers of the other two sisters.

Holzmann said neither parent had been informed, no consent forms had been signed, and the entire process had been carried out in secret.

The doctor, meanwhile, has reportedly claimed the full medical records were lost either in a "flood" or during a clinic "relocation."

The family has made a criminal complaint against the medic with Graz Public Prosecutor’s Office.

They accuse the doctor of serious fraud and have also sworn out a civil lawsuit seeking damages.

But the family say they are only making the case public to contact other possible victims.

Holzmann explained: "It can be assumed that the doctor also injected other patients with sperm from strangers without their knowledge."

(Simona Kitanovska / newsX)