Spanish judges ruled on Tuesday that a planned independence referendum in Catalonia was illegal, dealing a blow to its drive for self-rule.

The north-eastern region vowed to push on with the vote, however, defying fierce resistance from the national government, which has called for unity.

Under Spain's constitution a region "cannot unilaterally call a referendum on self-determination to decide on its integration in Spain," according to the ruling released by the Constitutional Court.

It ruled "unconstitutional and null" a declaration by the Catalan regional parliament which claimed Catalonia had a sovereign right to hold a vote on its future.

The court upheld a legal challenge to that declaration by the national government and said any "right to decide" by Catalans could only be exercised in accordance with Spain's 1978 constitution, which insists on the unity of Spain.

Leaders in Catalonia have called the referendum for November 9 to ask Catalans whether their region should be a separate, independent state.

Spain's conservative Prime Minister Mariano Rajoy has argued that Catalonia cannot hold a referendum like Scotland because Spain, unlike Britain, has a written constitution that rules out such a move.

Tuesday's ruling poses a setback for Catalan president Artur Mas, who has said he is determined to hold the vote legally.

But the Catalan government insisted shortly after the ruling that it would not derail the drive for a referendum.

"This will have no effect on the process," its spokesman Francesc Homs said on Catalan television.