Verdi's "Requiem" conducted by Daniel Barenboim was cancelled Friday because of a strike by La Scala's employees.
With apologies to the audience, La Scala announced that Friday's long-awaited "Requiem" performance was called off.
The concert was supposed to cap a year of commemorations marking the 50th anniversary of the death of Arturo Toscanini and one of Barenboim's first appearances in his new role as La Scala's principal guest conductor.
The theater did not indicate if the concert would be rescheduled.
Barenboim, who gave an interview to The Associated Press on Wednesday, declined to discuss the possibility of a strike. But the mood backstage at La Scala has been tense since union leaders announced their intention last week to strike Friday night's concert.
"A strike should not be discussed or considered by anybody outside those who negotiate until the moment that it either happens or doesn't happen," Barenboim told The AP during an interview in his private rehearsal area backstage on Wednesday, saying it had no impact on rehearsals.
Only Thursday did it become clear that there was no longer hope of reaching a deal in time to save the concert.
La Scala's management said in a statement that it had proposed on Tuesday calling in a mediator to overcome the impasse.
"The management of La Scala does not understand why the response to this proposal was negative. Nor does it see in the short-term any change on the horizon," the statement said.
La Scala said that it was prepared to offer a pay increase, based on the strong earnings in the last two seasons, noting that the number of concerts has risen from 164 a year in 2001 to 273 in 2007. But it said it was unable to negotiate a deal with its workers in the absence of a national contract, and no talks on a national level have been scheduled.
Musicians who spoke to The Associated Press after the rehearsal Thursday said La Scala has not raised wages in seven years, and that they have been pressing for months for resolution. Details of the negotiations were privileged.
"We are sorry for Maestro Barenboim," said one musician exiting Thursday's rehearsal, who refused to give his name because he is part of the union leadership.
Rehearsals have proceeded professionally despite the strike threat, he said, and that everyone had avoided the topic of a strike with Barenboim, who arrived in Milan on Oct. 21 to prepare for the "Requiem" as well as the 2007/08 season opener, Wagner's "Tristan und Isolde" on Dec. 7.
La Scala has 800 employees, including 135 musicians and 107 in the chorus; musicians said it was the fourth strike since 1989.
"We see tram drivers go one year without a contract and strike, and they get what they want," violinist Agnese Ferraro said. "This is our last resort."
La Scala has experienced management turmoil in recent years with the abrupt departure in 2005 of principal conductor Riccardo Muti after 18 years at the famed Milanese opera house.
La Scala's general manager Stephane Lissner has not replaced Muti, rather naming Barenboim as "Maestro of La Scala," a less formal arrangement under which the music director of the Berlin Staatsoper will produce at least two operas a year at La Scala through 2011.
The arrangement also includes co-productions with the Staatsoper.
La Scala also will have other guest conductors, including Daniele Gatti and Riccardo Chailly.
Music lovers Maria Penatti and Renata Riboni - both in their 80s - left the free public rehearsal of the "Requiem" in a state of joyous agitation.
"We're glad we came today," said Riboni. "I would have a ticket tomorrow, but with the threat of the strike I wasn't sure."
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