Spain's prime minister, Mariano Rajoy, has rejected a request by the leader of Catalonia to approve a referendum that would allow the north-eastern region to decide whether to secede from the rest of the country.

In a written reply on Saturday, Rajoy told Artur Mas "the ties that bind us together cannot be undone without enormous cost" and that the two politicians should hold talks instead.

His answer officially rebuffs the pro-separatist request Mas made in July.

Spain's constitution says only the central government can call a referendum. More than 1 million people showed support for Catalan independence last week by joining hands to form a 250-mile human chain across the region. Polls indicate about half of Catalonia's 7.5 million people support independence.