MADRID -  Catalonia's independence drive has sent dozens of firms fleeing as doubts grow over the economy of a region that, while one of Spain's richest, is also deeply indebted and dependent on trade with the EU and the rest of Spain.

The semi-autonomous region accounts for one fifth of national GDP and is Spain's largest exporter, but as the country's worst political crisis in a generation drags on there are fears businesses will seek more stable ground.

"To return to Catalonia you must be absolutely certain there is no instability," Juan Ignacio Sanz, banking expert at Barcelona's ESADE business school, told AFP.

"And that could take several years."

At least 40 large and medium-sized firms have already moved their legal headquarters out of the region, including Spain's third-largest lender Caixabank, energy giant Gas Natural and highway toll group Abertis.

Spain's Association of Registrars said Friday that a total of 540 businesses had sought to relocate their legal addresses from Catalonia from October 2-11.

Although no major job losses have yet been announced, Catalonia is set to see its tax revenues drop as businesses move out.

"If what has happened in recent weeks continues, that should make the Catalan government face reality: Big firms are leaving and that's catastrophic for Catalonia's tax revenues," Eric Dor, director of economics at the IESEG management school in Paris, told AFP.

Separatists argue that Catalonia is helping to prop up Spain's economy, paying more in taxes than it gets back, and they believe that breaking from Madrid would allow the region to prosper.

The region in northeast Spain is the country's most visited tourist area, a major industrial force and a leader in research and development, publishing and new tech.

But it is in debt to the tune of 76.7 billion euros ($90.6 billion) -- mostly owed to Madrid -- and it is unable to borrow directly on financial markets.

"It is inconceivable that (Prime Minister Mariano) Rajoy's government would simply wipe away this debt in the event of a Catalan secession from Spain," said Nathan Jones, a Spain expert and politics professor at Britain's Nottingham Trent University.

The crisis is also impacting Spain as a whole, with deputy prime minister Soraya Saenz de Santamaria warning on Friday that it "imperils" the country's recovery from the financial crisis.

Catalan president Carles Puigdemont insists an independent Catalonia would be a vibrant economic member of the European Union.

But Brussels has repeatedly said a breakaway Catalonia would automatically leave the bloc and would have to wait for admission -- and its incorporation efforts would almost certainly be vetoed by Madrid.

Dor said that were an independent Catalonia to enter the EU, its GDP per capita of 28,600 euros ($33,600) would rank it 12th among member nations.

"So it absolutely could be a viable state," he said.

But, Jones points out, there is "no certainty" that a debt-laden Catalonia would meet membership criteria -- including adopting the euro currency-- even if the political hurdles were overcome.

"All of this would potentially leave Catalonia isolated from key trading partners and heavily indebted to the Spanish government, severely weakening its economy," he said.

The Mobile World Congress, the phone industry's largest annual trade fair held every year in Barcelona, said it would hold its 2018 event in February as planned, after media reports suggested it was considering delaying.

"We are continuing to monitor developments in Spain and Catalonia and assess any potential impact," a spokeswoman told AFP.

Ricardo Mur, vice-president of the Aragon Business Confederation said his region, which borders Catalonia, had seen a surge in activity.

"Industrial estates are almost full due to the transfer of Catalan businesses," he told AFP.

After big firms, "medium and small business will follow... the decapitalisation of Catalonia is very significant and difficult to reverse," Mur said.

- 'Maximum concern' -

Ratings agency Standard and Poor's, which is considering downgrading Catalonia, said on Thursday that the region risked recession if the row over independence dragged on.

The CEOE business lobby warned Catalonia was already "seriously affected" by the crisis.

Puigdemont, who on Tuesday declared independence but suspended it pending talks with Madrid, told regional lawmakers that the flight of companies had "no real effect on our economy."

But analysts, including Juan Fernando Robles, economics professor at Madrid's UNED university, fear it will take years to restore investor faith.

"Foreign investment is reduced and therefore local investment suffers," he said.

"It will be a long time before confidence returns."