The Dallas Opera has cancelled a gala in which Plácido Domingo was due to perform after the growing number of sexual harassment allegations against the Spanish singer.

Twenty women have accused Domingo, the general director of the Los Angeles Opera, of sexually harassing them in incidents dating back to the late 1980s.

Domingo, 78, has described the accusations, first reported by the Associated Press, as “deeply troubling and, as presented, inaccurate”.

Late on Thursday, the Dallas Opera said it had decided to abandon a gala featuring Domingo next year. “In light of ongoing developments regarding allegations made against Plácido Domingo, the Dallas Opera has decided to cancel the March 11 2020 gala, in which he was scheduled to perform,” it tweeted.

Nine women – eight singers and a dancer – told the news agency last month they had been harassed by Domingo, prompting LA Opera to begin an investigation into the singer, who has been its general director since 2003.

Earlier on Thursday, the Associated Press (AP) reported that 11 more women had come forward with similar allegations, including a former Washington National Opera singer who said Domingo had groped her so hard during the 1999-2000 season that she cried out in pain.

Angela Turner Wilson said she had been felt compelled to speak out after Domingo reacted to the allegations by suggesting that “the rules and standards by which we are and should be measured against today are very different than they were in the past”.

Washington National Opera issued a statement saying it was “disturbed and disheartened”, while the Los Angeles Opera said it took the new allegations “extremely seriously” but could not discuss any specific claims because of the investigation.

Among the latest accusers are LA Opera backstage staff who said Domingo’s alleged behaviour was common knowledge and that management had been aware of it for decades. Some costume department employees also said they made it a point to keep young female singers away from him.

The Metropolitan Opera in New York said it would continue to await LA Opera’s findings before making any final decisions about Domingo’s future at the Met, where he is scheduled to perform starting on 25 September.

A spokeswoman for Domingo accused AP of conducting an “inaccurate [and] unethical” campaign against the singer and described the latest claims as “riddled with inconsistencies and, as with the first story, in many ways, simply incorrect”.

She added: “Due to an ongoing investigation, we will not comment on specifics, but we strongly dispute the misleading picture that the AP is attempting to paint of Mr Domingo.”