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 but three people told the Old Vic they had contacted the police, while 14 of the 20 complainants were told by the investigators that they should consider going to the police.
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.
With the exception of one of the claims, none of the reported incidents were raised formally or informally with management.
Spacey was invited to participate in the investigation but the Old Vic said he did not respond.
The investigation is continuing so more people can come forward.
Current artistic director Matthew Warchus said he had "genuine and deep sympathy for all those who have come forward".
Mr Warchus 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 said 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."
Mexican actor Roberto Cavazos, who acted in several plays at the theatre, had previously said Spacey "routinely preyed" on young male actors. The theatre said it could not comment on this due to confidentiality.
The Old Vic set up a confidential complaints process on 31 October 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.
Then, on 8 November, ex-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 under way.
Spacey's career has been hugely damaged by the allegations, with US network Netflix ending further production of House of Cards and the International Academy of Television Arts and Sciences announcing it will no longer give the actor a special Emmy award.
His agent and publicist have dropped him as a client.
He was also 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 was "taking the time necessary to seek evaluation and treatment".
They did not give any information about what kind of treatment he wanted.