LONDON — The Old Vic theater here said 20 people have come forward with allegations of inappropriate behavior by the actor Kevin Spacey before and during his time as the theater’s artistic director.

Following accusations of sexual misconduct against the actor, the theater opened an investigation into Mr. Spacey’s 11-year tenure, from 2004 to 2015, and released the conclusions in a report on Thursday. The theater apologized to those affected for “not creating an environment or culture where people felt able to speak freely.”

The report said that only one of the 20 complaints was raised at the time. The theater said that despite processes being in place for reporting such issues, people did not feel able to raise concerns.

The report said of Mr. Spacey: “The investigation found that his stardom and status at the Old Vic may have prevented people, and in particular junior staff or young actors, from feeling that they could speak up or raise a hand for help.”