Human error caused the Alton Towers rollercoaster crash in June that seriously injured five people, the Staffordshire theme park has said.
Two women – Victoria Balch and Leah Washington – had legs amputated after the Smiler ride hit an empty carriage on 2 June.
Staff misunderstood a shutdown message and wrongly restarted the ride, an investigation by the park confirmed.
No technical or mechanical issues were found with the ride itself.
The ride, which has been closed since the crash, will reopen next year with improved safety measures.
“A ride shutdown message was misunderstood by staff at the ride,” an Alton Towers spokeswoman said.
“This led to a decision to manually restart the ride, overriding the control system without appropriate safety protocols being followed correctly.”
A total of 16 people were injured when the carriage they were in collided with an empty one that had come to a halt ahead of them.
Merlin Entertainment, which owns Alton Towers, previously said all 16 would receive compensation.
Four people sitting in the front row were among those most seriously hurt.
Ms Balch, 20, from Lancashire, had six rounds of surgery in a bid to save her leg before needing an amputation.
Ms Washington, 17, from Barnsley, also had one of her legs amputated and her 18-year-old boyfriend Joe Pugh shattered his knees.
Daniel Thorpe, 27, from Buxton in Derbyshire, was treated for a collapsed lung and lower leg injuries.
Another passenger Chanda Chauhan, 49, from Wednesbury, had surgery after suffering internal injuries.
he Health and Safety Executive said its own investigation was “still ongoing”.
Alton Towers said it had followed “standard HR procedures and taken the appropriate action” when dealing with the staff whose errors caused the crash.
A spokeswoman said: “The outcome of this however remains a private matter between us and any individual concerned.”
Merlin Entertainments said significantly lower numbers of visitors to Alton Towers over the summer had prompted a restructuring of the business which could see 190 job losses following an 11.4% fall in revenues.
When the Smiler reopens next year, it will feature an additional level of authorisation so no manual override can happen without a senior member of staff, the theme park said.
“We are confident that lessons have been learned and that appropriate action has been taken to address all the issues raised by our investigation.”