Spaniards erupted in uproar on Wednesday over a call for this week's King's Cup final to be suspended and held behind closed doors in another stadium to thwart protests by Basque and Catalan separatists.
Madrid regional government president Esperanza Aguirre made the threat to scupper plans by some fans to whistle during the national anthem before Friday's match between Basque club Athletic Bilbao and Catalan giants Barcelona.
Fans who favour independence for Catalonia and the Basque Country are expected to whistle in protest at Spanish rule before the match gets underway at Madrid's Vicente-Calderon stadium.
"Insults to the flag or the anthem are a crime under the penal code," Aguirre told Onda Cero radio on Tuesday. "It should not be allowed and as such, in my opinion, the game should be suspended and held behind closed doors somewhere else."
The reaction on Wednesday was predictably heated, with the front page of the Barcelona-based sports daily Mundo Deportivo telling Aguirre: "Why Don't You Shut Up?"
"The Cup is being poisoned," declared fellow Catalan sport daily Sport while La Vanguardia, the leading Catalan-based daily, said Aguirre, a powerful figure in the conservative Popular Party, was isolated.
A palace official had expressed regret over the politicisation of the game and said the Prince Felipe would attend as scheduled in the absence of King Juan Carlos, who is recovering from an injury sustained in a widely-criticised elephant hunt.
Spanish opposition lawmakers meanwhile accused Aguierre of trying to deflect attention from Madrid's troubled finances, while others noted that the far-right Falange movement had been allowed to rally in the capital on cup final day.
Madrid's Superior Court has reportedly overruled attempts by the government to change the date of the fascist protest, which is expected to condemn Catalan and Basque independence aspirations.
Aguirre said she realised any suspension of the game would provoke a row but insisted the whistling could not be accepted, although she accepted that not all Barca or Bilbao fans were nationalist, separatist or "anti-Spanish".
"This is the championship of Spain," said Aguirre, who was born into a Spanish noble family and has the title Countess of Murillo.
"This cup was given by the president of the republic when there was a republic, Franco when there was Franco and now it is the cup of his majesty the king. But it is the Spanish Cup."
The row has led to an appeal for calm from interior minister Jorge Fernandez Diaz but Aguirre has been roundly condemned for trying to stifle free speech.
Santiago Espot, a Catalan promoter of the whistling protest, said Aguirre had "reawakened the ghost" of Jose Antonio Primo de Rivera, a Spanish dictator who ruled from 1923-30.
From the footballing world, Barcelona president Sandro Rosell was quoted as saying: "I want all Barcelona fans to be able to express their feelings freely."
Barca defender Gerard Pique added: "There are much more important problems than this in the country to worry about."