Inability of the defence to cross-examine a prosecution witness may arise through anonymity or absence, but the accused's right to a fair trial remains absolute: Al-Khawaja and Tahery v United Kingdom  ECHR 110 (20 January 2009).
The Chamber held (34) that Article 6 § 3(d), the defendant's right "to examine or have examined witnesses against him and to obtain the attendance and examination of witnesses on his behalf under the same conditions as witnesses against him",
"...is an aspect of the right to fair trial guaranteed by Article 6 § 1, which, in principle, requires that all evidence must be produced in the presence of the accused in a public hearing with a view to adversarial argument (Krasniki v. the Czech Republic, no. 51277/99, § 75, 28 February 2006). As with the other elements of Article 6 § 3, it is one of the minimum rights which must be accorded to anyone who is charged with a criminal offence. As minimum rights, the provisions of Article 6 § 3 constitute express guarantees and cannot be read, as it was by the Court of Appeal in Sellick ... [ EWCA Crim 651], as illustrations of matters to be taken into account when considering whether a fair trial has been held (see Barberà, Messegué and Jabardo v. Spain, 6 December 1988, §§ 67 and 68, Series A no. 146; Kostovski v. the Netherlands, 20 November 1989, § 39, Series A no. 166). Equally, even where those minimum rights have been respected, the general right to a fair trial guaranteed by Article 6 § 1 requires that the Court ascertain whether the proceedings as a whole were fair."
"Whatever the reason for the defendant's inability to examine a witness, whether absence, anonymity or both, the starting point for the Court's assessment of whether there is a breach of Article 6 §§ 1 and 3(d) is set out in Lucà, ... [Lucà v. Italy, no. 33354/96, ECHR 2001], at § 40:
"If the defendant has been given an adequate and proper opportunity to challenge the depositions either when made or at a later stage, their admission in evidence will not in itself contravene Article 6 §§ 1 and 3(d). The corollary of that, however, is that where a conviction is based solely or to a decisive degree on depositions that have been made by a person whom the accused has had no opportunity to examine or to have examined, whether during the investigation or at the trial, the rights of the defence are restricted to an extent that is incompatible with the guarantees provided by Article 6 [references omitted].""
Naturally a court will do all it can to counterbalance the adverse effect on the defence of witness anonymity or unavailability. Other material witnesses may be available, there may be expert witnesses who can assist with credibility issues, and the judge may warn the jury of the need to treat with care evidence that had not been subjected to cross-examination. Even so, such measures are not able to overcome the unfairness that arises where a conviction is based solely or to a decisive extent on the evidence of a witness who has not been cross-examined (37):
""Even when 'counterbalancing' procedures are found to compensate sufficiently the handicaps under which the defence labours, a conviction should not be based either solely or to a decisive extent on anonymous statements." [citing para 76 of Doorsonv. theNetherlands, judgment of 26 March 1996, Reports 1996 II]."
Here the Chamber found violations of Art 6 and awarded damages for the time the appellants had spent in prison.
Under the applicable domestic law here the critical criterion was the risk of unfairness that admission of the challenged evidence would give rise to. The evidence had been wrongly ruled admissible. What if the criterion for admission was the apparent reliability of the evidence? For example, what if the circumstances in which a hearsay statement was made were such as to present a reasonable assurance that it was a reliable statement? Would inability to cross-examine its maker give rise to unfairness if the statement was the sole or decisive evidence against the accused?
It is likely that a criterion for admission such as a reasonable assurance of reliability will be regarded as satisfying the requirement of fairness if, where it is met, it means that cross-examination could not have affected its probative value. See, for example R v Hovell  1 NZLR 610, (1987) 3 CRNZ 106 (CA), in which the police statement of an elderly rape complainant who had died before these proceedings was admitted to prove that there had been such an offence, committed by a person she could only vaguely describe. The description did not exclude the accused, but it was clear that if she had been asked whether the assailant was the accused she would only have been able to say she didn't know. The Court, with Cooke P (later, Lord Cooke) presiding, in a judgment delivered by Casey J, concluded that cross-examination here would not have made any relevant difference to her description of the assailant, and her hearsay statement was held to have been correctly ruled admissible. There could not, in this case, have realistically been any dispute over whether the rape had occurred as there was medical evidence corroborating the event and there was no reason why an elderly person should make a false assertion of that kind.
The absolute rule applied by the Strasbourg court is probably subject to an exception for cases where cross-examination could have no real effect; that issue did not arise for decision in the two (unrelated) cases that were jointly decided in Al-Khawaja and Tahery.
Update: Lord Brown has doubted that Al-Khawaja lays down an absolute rule: see his concurring judgment in R v Horncastle  UKSC 14 (9 December 2009). This case, awaited by the Grand Chamber’s Panel before its consideration of the United Kingdom’s request that Al-Khawaja be referred to the Grand Chamber, addresses the different measures to ensure trial fairness as between the continental legal systems and the common law tradition (now reflected in statutory provisions) in relation to the admissibility of hearsay evidence. The issue is hugely important, as Lord Brown emphasises at 113:
“These appeals are of the utmost importance. If the Strasbourg case law does indeed establish an inflexible, unqualified principle that any conviction based solely or decisively on evidence adduced from an absent or anonymous witness is necessarily to be condemned as unfair and set aside as contrary to articles 6(1) and 6(3)(d) of the Convention, then the whole domestic scheme for ensuring fair trials – the scheme now enshrined (as to hearsay evidence) in the Criminal Justice Act 2003 and (as to anonymous evidence) in the Criminal Evidence (Witness Anonymity) Act 2008 – cannot stand and many guilty defendants will have to go free. It is difficult to suppose that the Strasbourg Court has in fact laid down so absolute a principle as this and, indeed, one exception to it, at least, appears to be acknowledged: the fairness of admitting hearsay evidence from a witness absent as a result of the defendant's own intimidation. But if this is recognised (and, as others have pointed out, this exception itself involves difficulties of proof) why not recognise other exceptions too provided only and always that the procedures honour the ultimate imperative of a fair trial? That, after all, is the overarching principle for which the great bulk of Strasbourg jurisprudence on article 6 stands.”
See also, R v Couture, blogged here 19 June 2007.