A failure to comply with an express requirement to consult may make a decision vulnerable to challenge if the Court is of the view that a breach of that requirement defeats the purpose of the relevant legislation.
Where the decision-maker fails to undertake consultation that is required because of a promise it has made or its prior practice (procedural legitimate expectation), the Court may quash the decision where it is of the opinion that fairness requires it to do so and there are no overriding public interest reasons to justify defeating the legitimate expectation.
Where consultation is undertaken, but does not comply with the Gunning principles, a subsequent decision may be quashed by the Court, depending on the nature of the non-compliance and its effect. However, as is the case with all other grounds for judicial review, the Court must refuse to quash the decision (or award any other relief) if it appears to it to be highly likely that the outcome for the person affected by the decision would not have been substantially different if the decision-maker had not failed to comply with the procedural requirement in question (e.g. to consult fairly and properly). However, the safest course of action in all cases is to put systems in place to avoid any inadvertence or maladministration which could lead to mandatory consultation not being carried out, or being carried out unfairly or improperly.