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.
For years Catalonia has formally requested for this right to be exercised in different ways, and the Spanish government has made every effort to stop any possible way to channel this right. This has continued until today, when the Spanish militarized police has raided the Catalan institutions and arrested members of the Catalan government that was elected with the mandate to let Catalans exercise this right in spite of the official Spanish position, which it's trying to do through a referendum where both positions in favor and against independence may have equal opportunities to be heard.
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“.[0]
[0] http://www.icj-cij.org/files/case-related/141/16010.pdf