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Freiburg - A mother and her partner have been handed lengthy prison terms by a court in southern Germany for the years-long sexual abuse of the woman's child, whom they sold for sex with men on the dark net.

The 48-year-old mother will not appeal her 12-and-a-half-year prison sentence because she wanted to signal to her 10-year-old son that "things would be quiet now," her lawyer Matthias Wagner said.

Katja Ravat, the child's lawyer, said the mother had not shown remorse at any point during trial, adding that she "had hoped that [the defendant] would make a statement, issue a written apology to her child, or at least explains to him why she did it."

The woman's 39-year-old male partner was handed a 12-year prison term followed by "preventive detention," which could delay his release on the grounds that he poses a danger to the public.

The mother's partner, who already had a previous conviction of serious child abuse, confessed to the court in June that he was the "driving force" behind the crimes.

The 39-year-old admitted the 100-page charge sheet was accurate, except for a few minor details, saying that he had put pressure on his girlfriend to allow her son to be raped and abused by men they found online.

The Freiburg court also ordered the two abusers to pay 42 500 euros (49 200 dollars) in damages to the boy and a young girl, another victim in the case.

The mother confessed to the crimes during the trial but did not reveal her motive. Prosecutors say it was partly due to her relationship with her partner.

Six other offenders have been handed lengthy prison sentences as part of the case.

A Spanish national received a 10-year prison sentence on Monday after admitting that he had raped the boy on at least 15 occasions and had paid the mother and her partner more than 10 000 euros.

Freiburg police chief Peter Egetemaier said that the investigation into the couple had led them to around 20 other people who are now under investigation for child sexual abuse based on confiscated images and video footage.