"The given testimony shows that in all but one case, those individuals alleging inappropriate behaviour by Kevin Spacey did not come forward at the time or asked that no formal action be taken.
"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... 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 report said.
The allegations range "from behaviour that made people feel uncomfortable all the way through to sexually inappropriate behaviour", the theatre said.
Spacey, who was the Old Vic's artistic director between 2004 and 2015, already faces a police investigation in the UK over an alleged sexual assault in London.
Releasing the conclusion of its investigation, the Old Vic said in a statement on Thursday: "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."
Spacey has not yet commented on the allegations, and The Old Vic has said it has not been possible to verify the claims.
The theatre said having Spacey as its artistic director put it in a "unique position", and that the "cult of personality" surrounding his role may have prevented people, particularly junior staff or young actors, "from feeling they would speak up or raise a hand for help".
The theatre added: "This investigation concluded that there have been 20 personal allegations of incidents relating to The Old Vic ranging from 1995 to 2013, with the majority (all but two) falling before 2009.
"No legal claims, formal grievances, formal disputes, settlement agreements or payments made or authorised were made at all in relation to Kevin Spacey during his tenure.
"It has also not been possible to verify any of these allegations, and it is important to note that Kevin Spacey has not commented on them. The review cannot therefore make any findings of fact about the alleged misconduct."
The theatre engaged an external law firm, Lewis Silkin, 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. None of The Old Vic's trustees were aware of the allegations, they have said, adding that Spacey was invited to participate in the investigation but did not respond.
The investigation concluded:
• There are allegations of a range of inappropriate behaviour relating to Kevin Spacey during his time as Artistic Director of The Old Vic.
• No correspondents alleged they were minors at the time that their complaint arose. The Trustees were unaware of the allegations.
• With the exception of one of the claims, none of the reported incidents were raised formally or informally with management.
• Staff who observed this behaviour claimed they were unclear about how to respond; in some cases they did not consider allegations of misconduct to be serious and, where they clearly did, they said they did not feel confident that The Old Vic would take those allegations seriously given who he was.
• The following factors may have contributed to this escalation problem: star power; lack of clarity of The Old Vic's accepted code of behaviour; confusion regarding the impact of reporting misconduct; career insecurity; tribalism between operational and creative staff during his tenure causing information gaps.
• Calling out this behaviour could have been easier if there had been a non-formal and confidential mechanism in place by which complaints could be raised safely within the organisation.
