Spain's northeastern region of Catalonia has opened polling stations to hold an informal vote on independence that the central government has called illegal.

The regional Catalan government has pushed forward with the vote despite Spain's Constitutional Court ordering its suspension on Tuesday after it agreed to hear the Spanish government's challenge that the poll is unconstitutional.

The Catalan government expects polling stations to be manned by more than 40,000 volunteers in defiance of the court's suspension. Results aren't expected until Monday morning.

The ballot asks voters two questions: should Catalonia be a state, and if so, should it be independent.

Catalonia, with around 7.5 million inhabitants and Barcelona as its capital, is one of Spain's wealthiest regions.