The first paragraph of Article 273 of Directive 2006/112/EC does not require Member States to register of their own motion a taxable person for the purposes of collecting VAT solely on the basis of tax returns, other than those relating to VAT, where those returns would have made it possible to establish that the taxable person had exceeded the exemption threshold for VAT. The principles of legal certainty and protection of legitimate expectations do not preclude a tax authority from deciding that veterinary services are subject to VAT in circumstances such as those in the main proceedings, provided that that decision is based on clear rules and the practice of that authority was not such as to create in the mind of a reasonably prudent economic operator a reasonable expectation that VAT would not be applied to those services, that being a matter for the referring court to establish.