The Old Vic theatre says it has received 20 personal testimonies of alleged inappropriate behaviour by Kevin Spacey, who was its artistic director between 2004 and 2015.
Those affected said they “felt unable to raise concerns”, and he “operated without sufficient accountability”.
The London theatre said it “truly apologises” for not creating a culture where people felt able to speak freely.
The BBC has contacted Spacey’s legal representatives for comment.
The Old Vic’s announcement follows recent allegations of sexual harassment and predatory behaviour made against the double Oscar winner and former House of Cards actor while at the theatre.
Spacey has not responded to them.
The actor’s behaviour was alleged to have ranged from making people feel uncomfortable to sexually inappropriate behaviour, the Old Vic investigation found. No-one alleged rape.
There were 20 individual allegations and 16 were former staff, all of whom were men.
Lewis Silkin, the external law firm engaged by the theatre to conduct the investigation, said more than half the allegations were said to have taken place inside the Old Vic.
The investigation also found there was not widespread knowledge of Spacey’s alleged behaviour within the organisation, the law firm said.