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To act ultra vires is to act beyond one's powers. The term often appears in discussion of delegated legislation (see: Delegated legislation). Because a Statutory Instrument (SI) is issued under the authority of Parliament, a charge that one has behaved in contravention of an SI could not be refuted on the basis that the SI conflicted with some other point of common law. It may, however, be a defence to claim that the SI was issued ultra vires; that is, the Minister who issued it was acting beyond the powers conferred by the original Statute. In reality, SIs are rarely challenged this way, because very careful attention is paid to such matters when they are drafted. However, ultra vires need not be direct; a special kind of ultra vires defense is that a delegated order is oppressive; it could be argued that Parliament never intends to delegate power to anyone to make this kind of legislation.
A charge that an act is ultra vires may also be levelled against legal corporations (e.g., companies). Until quite recently it was assumed that if a company acted outside the terms of its memorandum (see: Memorandum of association) then it was ultra vires, and contracts formed in this way were void (see: Void contract). Recent changes to legislation have made this charge unsustainable; company directors may be liable for breach of contract even if the contract is strictly ultra vires.