The Supreme Court of Canada heard arguments Thursday about a Muslim woman who wants the right to wear a veil over her face while she testifies against two men she accuses of sexually assaulting her.

It's a case that has dragged through the courts for more than three years and one that has pitted the constitutional right of religion freedom against the rights of the accused to a fair trial.

The case involves a woman known only as N.S., who wants to wear her veil, called the niqab, as she testifies against the uncle and cousin she accuses of sexually assaulting her over several years as a child.

A niqab covers the face so that only the eyes can be seen.

The two accused argue that if N.S. wears her veil, it will violate their right to be able to face their accuser in court. They say they need to see the accused's face as she is testifying, so they can assess her demeanour, which they say is key to defending themselves.

On Thursday, Supreme Court Justice Morris Fish joined others on the bench in suggesting there isn't a single defence lawyer who would agree to allowing a witness to testify against their client without their face being clearly visible.

By revealing only the eyes, a niqab makes it difficult for lawyers and juries to read any nuances in facial expressions during testimony.

N.S. has said that she is not unwilling to testify without her niqab, but that her Muslim faith dictates that she wear it in public.

When the Court of Appeal for Ontario heard the case, it ruled that witnesses who wear the niqab must remove it on the stand -- but only if it has been shown that in that case, wearing it would truly jeopardize the accused's right to a fair trial.

It said that deciding how best to balance a women's right to express her religious beliefs, and the right of the accused must be done on a case-by-case basis.

"If a witness establishes that wearing her niqab is a legitimate exercise of her religious freedoms, then the onus moves to the accused to show why the exercise of this constitutionally protected right would compromise his constitutionally protected right to make full answer and defence," the court said.

The Appeal Court sent the case back to the preliminary inquiry judge, and ordered the judge to let N.S. make arguments about her religious beliefs. But she decided to take the case to the Supreme Court of Canada, which agreed in March to hear the appeal.

The final ruling will likely have a significant impact on future cases involving religious garments worn in courtrooms as well as other accommodation made for religious beliefs.