71However, by way of exception to that principle, acts tainted by an irregularity whose gravity is so obvious that it cannot be tolerated by the legal order of the European Union must be treated as having no legal effect, even provisional, that is to say, they must be regarded as legally non-existent. The purpose of this exception is to maintain a balance between two fundamental, but sometimes conflicting requirements with which a legal order must comply, namely, stability of legal relations and respect for legality (Commission v BASF and Others, cited in paragraph 70 above, paragraph 49, and Hoechst v Commission, cited in paragraph 70 above, paragraph 70).