The primacy of a defendant's absolute right to a fair trial is preserved in aspects of Canadian evidence law concerning the withholding of disclosure in the interests of national security: R v Ahmad, 2011 SCC 6 (10 February 2011). This is because the court can, in the event that absence of disclosure compromises the right to a fair trial, order a stay of proceedings.
The legislative scheme under consideration in Ahmad is ss 38 to 38.16 and 39 of the Canada Evidence Act, R.S.C. 1985, c. C-5. Section 38.14 provides:
Protection of right to a fair trial
38.14 (1) The person presiding at a criminal proceeding may make any order that he or she considers appropriate in the circumstances to protect the right of the accused to a fair trial, as long as that order complies with the terms of any order made under any of subsections 38.06(1) to (3) [permiting disclosure] in relation to that proceeding, any judgment made on appeal from, or review of, the order, or any certificate issued under section 38.13 [prohibiting disclosure].
(2) The orders that may be made under subsection (1) include, but are not limited to, the following orders:
(a) an order dismissing specified counts of the indictment or information, or permitting the indictment or information to proceed only in respect of a lesser or included offence;
(b) an order effecting a stay of the proceedings; and
(c) an order finding against any party on any issue relating to information the disclosure of which is prohibited.
The Court, in a unanimous judgment, held  in relation to a conflict between the interests of national security which require concealment of information and the interests of the defendant,
"Where the conflict is irreconcilable, an unfair trial cannot be tolerated.
Under the rule of law, the right of an accused person to make full answer and defence may not be compromised."
This requirement of trial fairness was the criterion for constitutional validity of the legislation . The important consideration was the flexibility of the legislated scheme :
" ... the net effect is that state secrecy will be protected where the Attorney General of Canada considers it vital to do so, but the result is that the accused will, if denied the means to make a full answer and defence, and if lesser measures will not suffice in the opinion of the presiding judge to ensure a fair trial, walk free. While we stress this critical protection of the accused's fair trial rights, we also note that, notwithstanding serious criticisms of the operation of these provisions, they permit considerable flexibility as to how to reconcile the accused's rights and the state's need to prevent disclosure."
Important here is the context in which a stay of proceedings may have to be considered. Usually the stay is described as a remedy of last resort, but here a stay of proceedings may be required even though not all the information has been disclosed to the judge who therefore could not say that it was necessarily the only appropriate remedy [34-35].
There is no obligation on the defence to assist the court (for example by undertaking not to disclose to the defendant information given by the prosecution to counsel, see ) to avoid the need to order a stay :
"... the defence is under no obligation to cooperate with the prosecution and if the end result of non-disclosure by the Crown is that a fair trial cannot be had, then Parliament has determined that in the circumstances a stay of proceedings is the lesser evil compared with the disclosure of sensitive or potentially injurious information."
The trial judge must be able to "conclude affirmatively"  that the right to a fair trial has not been compromised.
For my analysis of trial fairness in various leading appellate courts, click here.