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Consultancy can cover any subject. By purporting to be a consultant, you're effectively claiming to be an expert in a specialist subject. While you don't necessarily need qualifications to become an expert, they would clearly help and, aty the same time, offer a greter level of comfort to potential clients.
If you are taliking about general management consultancy, then you would need to have (and articulate) a thorough awareness and appreciation of managaement principles. What it would take for you to have this is open to interpretation. You could read every management book in Easons and become an expert without necessarily getting a formal qualification. However, it's likely that firms of management consultants considering employing you, would like to see some tangible form of expertise in the form of a relevant third-level qualification.