Is there an exception to informer privilege?

In his judgment, Martland, J., also endorsed the view expressed by Lord Esher in Marks v. Beyfus [(1890), 25 Q.B.D. 494 (C.A.)], that the rule of law may nevertheless be departed from if disclosure of the informant’s name is necessary or right to show an accused’s innocence. Whether this is regarded as an exception to informer privilege or an example of one important public policy superseding another appears neither here nor there. What is important is that the rule against disclosure of a “police-informer” is regarded as sacrosanct unless it can be shown that disclosure of the informant’s name is necessary to allow the accused to make full answer and defence.