Foreign states would likely not treat a purportedly seceding state as an independent state based on a referendum alone. Instead, they will look to the "political fact" of actual secession and the secession's legitimacy within the law of the state from which the allegedly new state seceded.
The Supreme Court of Canada's view of how secession would be recognized in international law is described in the Reference re Secession of Quebec, [1998] 2 SCR 217:
Although recognition by other states is not, at least as a matter of theory, necessary to achieve statehood, the viability of a would-be state in the international community depends, as a practical matter, upon recognition by other states. That process of recognition is guided by legal norms. However, international recognition is not alone constitutive of statehood and, critically, does not relate back to the date of secession to serve retroactively as a source of a "legal" right to secede in the first place. Recognition occurs only after a territorial unit has been successful, as a political fact, in achieving secession.
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... one of the legal norms which may be recognized by states in granting or withholding recognition of emergent states is the legitimacy of the process by which the de facto secession is, or was, being pursued. The process of recognition, once considered to be an exercise of pure sovereign discretion, has come to be associated with legal norms. See, e.g., European Community Declaration on the Guidelines on the Recognition of New States in Eastern Europe and in the Soviet Union, 31 I.L.M. 1486 (1992), at p. 1487. While national interest and perceived political advantage to the recognizing state obviously play an important role, foreign states may also take into account their view as to the existence of a right to self-determination on the part of the population of the putative state, and a counterpart domestic evaluation, namely, an examination of the legality of the secession according to the law of the state from which the territorial unit purports to have seceded. As we indicated in our answer to Question 1, an emergent state that has disregarded legitimate obligations arising out of its previous situation can potentially expect to be hindered by that disregard in achieving international recognition, at least with respect to the timing of that recognition. On the other hand, compliance by the seceding province with such legitimate obligations would weigh in favour of international recognition. The notion that what is not explicitly prohibited is implicitly permitted has little relevance where (as here) international law refers the legality of secession to the domestic law of the seceding state and the law of that state holds unilateral secession to be unconstitutional.
