canada
This question was directly addressed by the Supreme Court of Canada in R. v. N.S., 2012 SCC 72. The issue question was "when, if ever, a witness who wears a niqab for religious reasons can be required to remove it while testifying."
The Court rejected both a rule that a witness would always be allowed to wear a niqab and a rule that a witness would never be allowed to wear a niqab. Instead, "the answer lies in a just and proportionate balance between freedom of religion on the one hand, and trial fairness on the other, based on the particular case before the Court" (para. 31). The assessment must be case-by-case, following a four-stage analysis:
1. Would requiring the witness to remove the niqab while testifying interfere with her religious freedom?
This is not an inquiry into what Sharia law requires, but instead relates to whether the person has "sincere religious reasons" for wishing to wear the niqab while testifying (para. 11). "The value of adherence does not depend on whether a religious practice is a voluntary expression of faith or a mandatory obligation under religious doctrine" (para. 36).
2. Would permitting the witness to wear the niqab while testifying create a serious risk to trial fairness?
Some witness evidence is uncontested, and credibility and cross-examination are not really at issue. In these cases, "being unable to see the witness's face will not impinge on the accused's fair trial rights" (para. 28).
3. Is there a way to accommodate both rights and avoid the conflict between them?
The parties should consider reasonably available alternatives that would "conform to the witness's religious convictions while still preventing a serious risk to trial fairness" (para. 33).
4. If no accommodation is possible, do the salutary effects of requiring the witness to remove the niqab outweigh the deleterious effects of doing so?
Here, the Court listed various factors that would assist in this weighing: how important the practice is to the claimant; the chilling effect on other potential witnesses/complainants if they know that to testify will require them to remove their niqab; the extent to which cross-examination and credibility of this witness is central to the case; the nature of the proceeding; the nature of the evidence to be given (paras. 34–45).
All of this would be argued and decided in a voir dire before the witness is called to give their evidence.