Self-determination of peoples is a jus cogens law and a cardinal principle in international law. It's part of the UN's Charter and it's enshrined in many international treaties subscribed by Spain and by the European Union.

The Spanish constitution recognizes the right to self-determination inasmuch as it explicitly enshrines the international treaties that define it.

Spain has always defended that this right to self-determination is confined to colonies because it would otherwise conflict with the also very important principle of territorial integrity. However, the International Court of Justice explained in its Advisory Opinion of 22 July 2010 regarding the case of Kosovo that “the scope of the principle of territorial integrity is confined to the sphere of relations between States”. Thus, even unilateral declarations of independence do not conflict with the aforementioned principle: “general international law contains no applicable prohibition of declarations of independence“.