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 acclaimed London theatre said it “truly apologises” for not creating an environment or culture where people felt able to speak freely.
It will “commit to a new way forward”.
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 theatre’s 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”.
The investigation is continuing so more people can come forward.
With the exception of one of the claims, none of the reported incidents was raised formally or informally with management.
Three people told the Old Vic they had contacted the police.
Spacey’s behaviour is alleged to have ranged from making people feel uncomfortable to sexually inappropriate behaviour. No one alleged rape.
There are 20 individual allegations and 16 are former staff, all of whom are men.
Lewis Silkin, the external law firm engaged by the theatre to conduct the investigation, said there was not widespread knowledge of the allegations.
Spacey was invited to participate in the investigation but the Old Vic said he did not respond.
The BBC has contacted his legal representatives for comment.
Current artistic director Matthew Warchus said he had “genuine and deep sympathy for all those who have come forward and said they were hurt in some way by my predecessor’s actions”.
He added: “Everyone is entitled to work in an environment free from harassment and intimidation. The Old Vic is now actively engaged in the process of healing and the process of prevention.”
‘Unfair to say everybody knew’
He described the allegations as “a shock and a disturbing surprise to many of us”.
“It is incorrect, unfair and irresponsible to say that everybody knew. But as a result of the investigation, what we have learnt is how better to call out this behaviour in future.”
He added the findings would help “our industry as a whole, as together we rapidly evolve an intelligent new standard of protection and support in and around the workplace”.
The theatre’s executive director Kate Varah added that they did not want to “just rush out a statement”, saying: “We have not slept since this came out because we have been doing it in a robust, careful way.”
On 31 October, the Old Vic set up a confidential complaints process for people involved with the theatre.
On 3 November police in the UK confirmed they were investigating an alleged assault on a man from 2008.
Former television news anchor Heather Unruh told a press conference in Boston that her son had been sexually assaulted by Mr Spacey, at the age of 18 in a bar in Nantucket, Massachusetts, in July 2016.
She said a criminal investigation was now under way.
“Mexican actor Roberto Cavazos, who acted in several plays at the theatre said Spacey “routinely preyed” on young male actors. The theatre said it could not comment on this due to confidentiality.
Spacey replaced in upcoming film
Spacey’s career has been hugely damaged by the allegations, with US network Netflix ending further production of House of Cards, while the International Academy of Television Arts and Sciences announced it will no longer give the actor a special Emmy award. His agent and publicist have also dropped him as a client.
He was also been removed from upcoming film, All the Money in the World, which had already been shot. He will be replaced by Christopher Plummer in the role of late oil tycoon John Paul Getty.
Spacey said on 2 November he was seeking treatment, in the wake of the allegations.
A representative for the actor said he “is taking the time necessary to seek evaluation and treatment”.
They did not give any information about what kind of treatment he wanted.