The Hungarian State Opera has come up with a dubious way around a stipulation that George Gershwin’s opera Porgy and Bess be performed by an all-black cast: it is allegedly asking its white, Hungarian singers to sign a paper saying they identify as African-American.

The company first put on the opera a year ago, leading to a spat with the Gershwin estate, which stipulates the opera should only be performed by a black cast.

The show is back for another run of six performances, the first of which was performed on 5 April. Hungarian media have speculated that the opera house may be using illicitly made photocopies of the scores that were rented for the original performances last year, as they do not have permission to play the show with a non-black cast.

The Hungarian website Index reported that before the 5 April performance, singers were asked to sign a paper that read: “I, the undersigned, hereby state that African-American origin and identity are an inseparable part of my identity. Because of this I am especially glad to be able to perform in George Gershwin’s opera, Porgy and Bess.”

The source told Index most singers had signed the paper and that while the management saw it as a joke, some people were worried that not signing it could affect their future at the opera house.

Programmes on sale in the foyer had a red notice emblazoned on them stating: “The manner in which this production of Porgy and Bess is being produced is unauthorised and is contrary to the requirements for the presentation of the work.”

The opera house’s contention is that in a country such as Hungary, the all-black cast rule essentially makes the work impossible to perform. The argument has won sympathy among many Hungarians. The debate has also become part of the culture wars over liberalism, a dirty word in Viktor Orbán’s nationalist Hungary.

The general director of the Hungarian State Opera, Szilveszter Ókovács, declined to comment on the alleged request to singers, referring the Guardian to answers he had given to Index, which in fact were a series of his own questions about black identity.

They included “What colour is ‘black’ on the Pantone scale?”; “One of Barack Obama’s grandparents was ‘white’, do you think it would be right if he performed in Porgy and Bess?”; and “Would you remain calm if you paid for a product displayed in the shop window, but the shop assistant informed you that you cannot buy it because you are not black?”

The opera house was in the news last year when it cancelled 15 performances of the musical Billy Elliot after rightwing media claimed it could “turn children gay”.