Child welfare authorities don't owe parents a duty of care, Canada's top court ruled Friday, saying such an obligation would put the treatment of children at risk by creating a conflict of duties.

The unanimous Supreme Court of Canada ruling marks the end of a lengthy battle by a southwestern Ontario family for millions of dollars in damages.

They claimed a treatment centre and social worker owed them a responsibility and had been negligent by depriving them of a relationship with their daughter.

But the court ruling found there can be no dual responsibility because it would hinder the care of the children. The system's paramount duty is to do what is best for the child, the ruling says.

"To recognize such a legal duty to the family of a child in their care would pose a real risk that a secure treatment centre and its employees would have to compromise their overriding duty to the child," the ruling says.

In 1995, the family's 14-year-old daughter, identified in the case as R.D., was taken from their custody after she wrote a short story for school claiming her parents sexually and physically abused her.

The parents claimed the allegations were false and that she had a history of mental illness. No criminal charges were ever laid and the case never went to trial.

The family filed a lawsuit against a case worker and the treatment centre asking for $40 million in damages. 

The suit alleged the centre treated R.D. as if she were a sex abuse victim even though she wasn't and failed to try to reintegrate the teen into her family.

They argued the interference caused them emotional distress and nervous shock for having been deprived of a relationship with the girl.

The girl had been sent to a facility for mentally disturbed teens, the Syl Apps Secure Treatment Centre, after three suicide attempts. She later agreed to become a ward of the state, despite her parents' objections.

When the family filed the lawsuit, Syl Apps tried to have it tossed out. In 2006, the Ontario Court of Appeal quashed their attempt and the treatment centre and social worker then appealed to the Supreme Court.