Auschwitz-Birkenau state museum has urged Turin authorities to intervene amid mounting controversy over the participation at the city’s international book fair of a publisher said to have close links with the Italian neo-fascist party CasaPound.

The museum has been participating in the book fair, which gets underway on Thursday, since 2015 but says it will pull out if the Altaforte publishing company remains among the exhibitors.

“Survivors cannot be asked to share space with those who question the historical facts that led to the Holocaust and who try to revive fascism in society,” the museum wrote in a letter to Turin’s council that was signed by its director, Piotr Cywiński, the Holocaust survivor and writer Halina Birenbaum, and Michele Curto, a former city councillor who set up a project to teach high school students about the Holocaust.

“This is not, as some have simplified, about a contract with a publishing house, but the highest values of democratic institutions, their vigilance and the Italian constitution, which exceed any contract,” the letter continued.

It follows the resignation from the fair’s organising committee of the Italian writer and teacher Christian Raimo, as well as planned boycotts of the event by the Italian authors group Wu Ming, the historian Carlo Ginzburg, the cartoonist Zerocalcare and the journalist Francesca Mannocchi.

“In Turin an additional step has been taken towards the acceptance of the new black shirts on the Italian political and cultural scene,” Wu Ming wrote on its website on Sunday.

Turin’s mayor, Chiara Appendino, insists the city is against fascism. Photograph: Marco Bertorello/AFP/Getty Images

Organisers of the three-day event have stuck by their decision to treat Altaforte, which is the publisher of a new book about Matteo Salvini, the Italian deputy prime minister and the leader of the far-right League party, the same as any other paying client.

In a statement on Facebook, the organising committee described itself as an ambassador of the Italian constitution and cited a clause within it stipulating that “everyone has the right to freely express their thoughts with speech, writing and any other means of communication”.

It is unclear how Turin authorities will respond to the letter from the Auschwitz-Birkenau state museum but the mayor, Chiara Appendino, a politician from the Five Star Movement, which is governing nationally alongside the League, said on Monday that Turin was “antifascist”. “There are no viable alternatives to this position,” she wrote on Facebook, adding the local council would continue to support the book fair.

“We will not abandon it because ideas are fought with stronger ideas … it is only with culture that we can put an end to every possible degeneration, extremism or return of what must be archived forever.”

In an interview with La Stampa on Tuesday, Francesco Polacchi, the president of Altaforte, said: “I have not questioned the existence of the Holocaust but I will not give up the participation in the book fair.” Polacchi added that “a little bit of dictatorship is good”, praised Benito Mussolini as “the best Italian statesman” and said fascism had helped to “reconstruct” the nation after the first world war.

Founded in 1988, the Turin event is one of the largest book fairs in Europe, attracting around 1,400 exhibitors each year.