Via Deutsche:
Why is the referendum illegal?
The Spanish Constitution states that Spain cannot be broken up. The Article 2 in the preliminary part of the Constitution states: ” The Constitution is based on the indissoluble unity of the Spanish Nation, the common and indivisible homeland of all Spaniards”. The Spanish Constitutional Court suspended the law passed by the Catalan parliament at the start of September to organise the referendum.
What is needed to change the Spanish Constitution?
Article 167: For an ordinary reform of the Constitution you need:
A three-fifth majority in each house of Parliament.