OTTAWA — The Supreme Court of Canada has set aside the acquittal of a man charged with sexually assaulting a developmentally challenged woman, and ordered a new trial.

The case centres on whether the woman, who has the mental age of a three- to six-year-old, was competent to testify against the accused.

At a preliminary inquiry, she was able to answer questions about her family life and school, but she struggled with questions about truth, promises and the consequences of lying.

The trial judge said she wasn't competent and the Ontario Court of Appeal agreed.

But in a 6-3 decision, the high court says the judge set the bar too high.

The majority said witnesses with mental disabilities may testify as long as they can understand and answer the questions and offer an oath or at least a promise to tell the truth.

"Sexual assault is an evil. Too frequently, its victims are the vulnerable in our society -- children and the mentally handicapped," Chief Justice Beverley McLachlin wrote in the ruling.

"Yet rules of evidence and criminal procedure, based on the norm of the average witness, may make it difficult for these victims to testify in courts of law. The challenge for the law is to permit the truth to be told, while protecting the right of the accused to a fair trial and guarding against wrongful conviction."

The minority argued that standard is too low.

The now 26-year-old woman, whose identity is protected by a publication ban, was allegedly repeatedly sexually assaulted by her mother's partner at the time. She was 19 years old when the alleged assaults occurred.

The woman told her special-education teacher about a "game" that she and her mother's former partner used to play together, which involved the man touching her. She later repeated this statement to police, indicating the man touched her breasts, genital area and buttocks underneath her pyjamas. She also said it happened many times.

Advocacy groups welcomed the high court's decision.

"It means that the voices of women with mental disabilities will be heard in court, and that they will be treated like all other witnesses are treated," said Joanna Birenbaum of the Women's Legal Education and Action Fund, one of the interveners in the Supreme Court case.

"Really, the ruling simply means that persons with mental disabilities are not required to meet a more onerous test than any other witnesses before they are allowed to enter the courthouse door and take the stand."

In its legal brief, the Women's Legal Education and Action Fund noted 83 per cent of women with disabilities will be sexually abused in their lifetimes, while 40 to 70 per cent of girls with intellectual disabilities will be sexually abused before the age of 18.

"It will mean, hopefully, that the pervasive sexual abuse of persons with mental disabilities will come more frequently before the courts," Birenbaum said of Friday's Supreme Court's ruling.

"So that persons with mental disabilities report sexual abuse, that it will be treated even more seriously by police and by prosecutors, with some assurance that when their evidence comes before the court, it will be accepted and heard."