M v Commissioner of Police of the Metropolis [2007] EWCA Civ 1361, [2007] All ER (D) 368 (Dec)
When taking a decision of whether or not to prosecute, no duty of care is owed to victims, even where the decision took into account the interests of the victims. It would not be reasonable to impose such a duty, since prosecutors must have regard to the wider public interest, as well as the interests of victims (which may not coincide with the public interest or the interests of other victims) and all other relevant circumstances.
Moreover, so far as the police are concerned, the effect of imposing such a duty might be that investigations would be carried out defensively and resources would be diverted from the most important function of the police, that of the suppression of crime.
