A Spanish audit office has ordered former Catalan leader Artur Mas and nine others to repay €4.9 million of public money spent on a non-binding independence ballot in 2014, the office said on Monday.

The 2014 vote was a symbolic ballot by pro-independence campaigners that was declared illegal by Spain's Constitutional Court and came three years before Mas's successor Carles Puigdemont held an independence vote last year.

The informal ballot was held after the government blocked a more formal vote which Catalan leaders had been pushing for. 

Mas was cited by the audit office - which oversees spending by political parties and within the public sector - as being the main responsible party for the payment of the fine

Some 80 per cent of the around 2.2 million voters backed secession, much higher than the usual around 50-50 split amongst Catalans on independence.

Mas was cited by the audit office - which oversees spending by political parties and within the public sector - as being the main responsible party for the payment of the fine, the office said.

Mas was barred from public office for two years in March 2017 for staging the informal referendum, however has continued to speak for the region's separation from the rest of the country.

In October 2017, the Catalan regional assembly held another ballot on the wealthy northwestern region's separation from the rest of Spain before declaring it independent.

The declaration prompted Madrid to take control of Catalonia and arrest warrants to be issued for many of the organisers.

Puigdemont fled the country after the central government took over and is currently in self-imposed exile in Brussels while many of his co-conspirators are in jail awaiting trial for their part in the secession push.

The ruling comes shortly before Spain's Supreme Court begins trials of nine leaders of the independence movement in which prosecutors are seeking up to 25 years on rebellion charges.