This is becoming more and more of a problem for Spain because it is setting itself up for a true constitutional crisis. Basically

It is also peculiar that the `national transition process' was initiated and some institutions for an independent state were created without waiting for the results of such a referendum. Moreover, the purpose and questions of the referendum clash with the Spanish Constitution which explicitly states the `indissoluble unity of the Spanish Nation' and received 91 per cent support in Catalonia in 1978. Claims of Catalonia being a sovereign entity have been unanimously rejected by the Spanish Constitutional Court. While the British Parliament validated the organisation of the Scotland independence referendums, the Spanish Parliament rejected the Catalan one by a very large majority.

(Euro Crisis in the Press LSE blog)

The Constitution says:

Article 2

The Constitution is based on the indissoluble unity of the Spanish nation, the common and indivisible country of all Spaniards; it recognises and guarantees the right to autonomy of the nationalities and regions of which it is composed, and the solidarity amongst them all.

Also

Article 92

1. Political decisions of special importance may be submitted to all citizens in a consultative referendum.

This is being interpreted restrictively to mean you cannot have a referendum in Catalonia only.

A society committed to the notion that government is always bad will have bad government. And it doesn't have to be that way. — Paul Krugman