The Old Vic theatre in London has said 20 people have claimed they were victims of "inappropriate behaviour" by Kevin Spacey.
The theatre released the conclusion following its investigation into allegations concerning the actor, who was its artistic director between 2004 and 2015.
The Old Vic said in a statement: “Despite having the appropriate escalation processes in place, it was claimed that those affected felt unable to raise concerns and that Kevin Spacey operated without sufficient accountability.
“This is clearly unacceptable and The Old Vic truly apologises for not creating an environment or culture where people felt able to speak freely.”
The theatre engaged an external law firm to investigate claims made against
Spacey, and has concluded that, with the exception of one of the claims, none of the reported incidents were raised either formally or informally with management.
During a press conference, it was revealed there were 56 responses to a confidential email address set up by the theatre.
The responses included messages from concerned members of the public, former staff and people who had attended events at the theatre.
Of the 56 responses, 20 were personal testimonies alleging inappropriate conduct by Spacey between 1995 and 2013.
He was artistic director between 2004 and 2015.
All but two of the allegations against Spacey pre-date 2009 and 16 of the 20 personal testimonies were from former members of staff.
The Old Vic chairman Nick Clarry said:
“On behalf of The Old Vic Board of Trustees, 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.”
Allegations against Spacey emerged last month when actor Anthony Rapp claimed Spacey had placed him on a bed and climbed on top of him following a party at his apartment in 1986.
Others have come forward with allegations against the actor, who is being investigated by police in the UK over an alleged sexual assault in Lambeth, south London, in 2008.