In a country famous for industrial action, an unlikely group of Italian employees is proving even keener to down tools than train drivers, rubbish collectors and car workers.

Ballet dancers and chorus members at La Scala in Milan have walked out over a series of protests described as surreal, causing the last-minute cancellation of the opening night of the theatre's ballet season on Wednesday.

The heavily unionised dancers are fighting the use in a production of Romeo and Juliet, pictured, of a stage with a 10-degree slope which they say gives them backache, while chorus members are refusing to obey the choreographer's request that they move in time to the music.

"These performers are not machines," said Giancarlo Albori, an official with the Italian CGIL union.

The behind-the-scenes drama, which rivals any of those performed on stage at the venerable theatre this year, kicked off when 16 of the 74 chorus members were asked to climb up on stage and sing in costume. "We are talking about 20 minutes during an hour and 45 minute performance in which they needed to synchronise body movements with the dancers," said a spokesman for La Scala.

The chorus demanded a €600 (£488) bonus per performance for each of the 16, and €400 for the other singers, a request that was turned down, he said. "They are on stage regularly in operas like Rigoletto and Lohengrin, as well as in some ballets, but they claimed it was not in their contract for ballets."

Albori countered that going on stage also meant singing from memory, without a score. "They were given no time for rehearsing," he said. The tilted stage, he added, was downright dangerous, a claim challenged by the spokesman who said it had already been used in a production in Paris.

The strike is the latest in a long line of such protests by Italian opera and ballet stars, who have fought to defend bonus payments for extra duties such as wielding fake weapons on stage and singing in foreign languages.

When performers at La Scala threatened to strike in 2008 to get a bigger share of the theatre's total wage packet technicians and ticket sellers took over the theatre dressed as vampires and threw fake money from the boxes in protest.

"This is the theatre of the absurd," stated Italian daily Corriere della Sera on Tuesday in a front-page article about the latest strike . "Today there is a payment for leaning over. Next they will be asking extra for putting on a leotard or standing on tiptoes."

The strike comes as generous funding to Italy's 14 opera houses is reduced. La Scala is one of the few to break even, but it has lost €7m in public subsidy this year thanks to austerity cuts.

"Without offending the nobility of art, is anyone looking at what is going on outside La Scala and feeling ashamed at these surreal events?" asked Corriere della Sera.

The theatre spokesman said problems behind the scenes at La Scala were usually ironed out at the last minute, just in time for opening night. "By the first performance things are perfect – it is a characteristic of La Scala and maybe Italy," he said. "But this time the demands made by performers were just too high."