The Old Vic says it has received 20 allegations of inappropriate behaviour made against former artistic director Kevin Spacey.
In a statement, the London theatre said it "truly apologises" for "not creating an environment or culture where people felt able to speak freely" and announced a "commitment to a new way forward".
It comes following an investigation into Spacey's conduct while he was artistic director at the venue between 2004 and 2015.
The investigation resulted in "20 personal testimonies shared of alleged inappropriate behaviour" in relation to the Old Vic, the theatre said.
Only one of the claims was reported, it said, but staff were "unclear about how to respond".
"During his tenure, the Old Vic was in a unique position of having a Hollywood star at the helm around whom existed a cult of personality," the statement said.
"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.
"The Old Vic is deeply dismayed by the allegations and thankful to all those who have contacted its helpline directly. Without them it would not have been possible to conduct such a thorough review."
The first allegation against Spacey was made by actor Anthony Rapp, who accused him of sexual assault .
Spacey said he did "not remember the encounter" but offered "the sincerest apology" if it had taken place.
He has not been charged with any crime.
The Old Vic said that, in relation to its investigation, "no correspondents alleged they were minors at the time that their complaint arose".
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Old Vic chairman Nick Clarry said: "To all those people who felt unable to speak up at the time, the Old Vic truly apologises.
"Inappropriate behaviour by anyone working at The Old Vic is completely unacceptable.
"We will foster a safe and supportive environment without prejudice, harassment or bullying of any sort, at any level. We want anyone who works here to feel confident, valued and proud to be part of the Old Vic family."
The current artistic director of the company, Matthew Warchus, expressed "genuine and deep sympathy for all those who have come forward".
"These allegations have been a shock and a disturbing surprise to many of us," he said.
The Old Vic's statement comes after Netflix and Sony Pictures cut ties with Spacey, cancelling his show House Of Cards and replacing him in an upcoming film .