The question will be: "Do you want Catalonia to be an independent state in the form of a republic?"

Spain is currently a constitutional monarchy, and Catalonia is one of its wealthiest regions.

Surveys suggest Catalan voters would narrowly reject independence, though most favour holding a referendum.

The central government in Madrid has repeatedly stressed that secession would violate the constitution. It is not clear if the referendum will go ahead.

In November 2014 Catalonia held a non-binding vote, called a "consultation", in which 80% of those who cast ballots backed independence.

However, just 2.3 million out of 6.3 million eligible voters took part.

Spain's Constitutional Court had ruled it illegal. But the secessionists viewed it as a defining moment.

In 2015, Catalan nationalist parties won an absolute majority in the 135-seat regional assembly and voted to start the process towards independence.