Ultra Vires

'Ultra vires' is Latin for 'beyond the powers'. In public law, a decision-maker only has the powers Parliament (or, for a company, its constitution) has actually given it; if it exceeds those powers, the decision is unlawful. The Court of Appeal has explained the link between this doctrine and ordinary judicial review grounds directly: an error of law by a public body can, in public law terms, be described as acting ultra vires.

Because an ultra vires decision is treated as one the decision-maker never had the legal power to make, it can be quashed by judicial review even years after it was made, and even where a statute purports to exclude the court's supervisory jurisdiction (an 'ouster clause') — the courts have historically read such clauses narrowly so as not to protect decisions that are legally invalid.