Barcelona, Spain • The crisis over Catalonia’s quest for independence escalated Thursday, as Spain’s central government prepared the unprecedented step of stripping the wealthy region of some of its self-governing powers after its leader refused to abandon secession.

In his latest display of brinkmanship, Catalan President Carles Puigdemont sent a letter to Spanish Prime Minister Mariano Rajoy just minutes before a deadline set by Madrid for him to backtrack on his calls to secede.

Puigdemont didn’t back down, however, and threatened to go ahead with a unilateral proclamation of independence if the government refuses to negotiate.

“If the State Government persists in blocking dialogue and the repression continues, the Parliament of Catalonia will proceed, if deemed appropriate, to vote on the formal declaration of independence,” Puigdemont’s letter said in an English translation provided by the Catalan regional government.

Spain’s government responded by calling a special Cabinet session for Saturday when it will set in motion Article 155 of the Spanish Constitution. That article allows for central authorities to take over all or some of the powers of any of the country’s 17 autonomous regions.

Regarded as the “nuclear option,” such a punitive measure takes the standoff to another level. It probably will trigger outrage in Catalonia and could backfire by fostering sympathy for the independence movement, which polls suggest is supported only by about half of Catalans.

With a mood of defiance hardening in the Catalan capital of Barcelona and the Madrid-based government adamant that the constitution doesn’t allow for the breakup of Spain, there seems to be no end in sight for one of Europe’s long-simmering disputes.

The standoff has intensified since Oct. 1, when Catalan authorities held an independence referendum that Spain’s Constitutional Court declared illegal. The national government sent thousands of police to enforce a court order disallowing the balloting, bringing violent clashes that further soured relations.

The dispute is increasingly encroaching on the European Union’s political agenda. Catalonia wasn’t officially to be discussed at an EU summit starting Thursday in Brussels, but leaders offered their views. French President Emmanuel Macron reiterated his recent support for Rajoy, saying that the summit would be “marked by a message of unity around member states amid the crises they could face, unity around Spain.”

While polls indicate that Catalonia’s 7.5 million residents are roughly divided over independence, an overwhelming majority wants to settle the issue in a binding legal referendum. Many Catalans have long stressed the region’s differences from the rest of Spain. The latest surge for independence began in 2010, when the Constitutional Court struck down key parts of a groundbreaking charter that would have granted greater autonomy for Catalonia and recognized it as a nation within Spain.

Article 155 has never been used in the four decades since democracy was restored after the dictatorship of Gen. Francisco Franco. The article leaves it up to the national government to decide what specific measures to take. Officials say Madrid will almost certainly seize control of Catalonia’s regional police to ensure law and order is maintained, along with tightening its grip on the region’s finances.

Other measures being mulled are removing Puigdemont’s presidential powers, rescinding regional control over education and schools, calling fresh elections that would dissolve the regional parliament, and taking control of public media that are seen as mouthpieces for Catalonia’s pro-independence ruling coalition.

Puigdemont claims the referendum gave him a mandate to declare independence. His government says more than 40 percent of 5.5 million eligible voters cast ballots, with most favoring independence.

So far, however, Puigdemont has only called for negotiations with Spain and international mediators.

Andrew Dowling, an expert in Catalan history at Cardiff University in Wales, said any declaration of independence in the Catalan parliament would be merely symbolic without border and institutional controls.

Such a unilateral declaration “will see (a) fracture between hard-liners and the pragmatic people in Catalonia, who are already seeing economic fallout,” Dowling said.

Spain’s Association of Commercial Registers said Thursday that 971 companies, including Catalan banks, multinationals and midsized businesses, have moved their registered addresses out of the troubled region because of concerns about its future.