"The investigation concluded that the incident was the result of human error culminating in the manual override of the ride safety control system without the appropriate protocols being followed," an Alton Towers spokesperson said following an internal investigation.
"The investigation also identified areas where protocols and the training of employees should be improved. There were found to be no technical or mechanical problems with the ride itself."
The company, which will make multi-million pound payouts to the women, said the ride will re-open at some point in 2016.
The closure of Alton Towers after the accident and the rollercaoster - one of its key attractions - significantly impacted Merlin, issuing a profit warning in the aftermath, as customers avoided theme parks after the re-opening.
Shares in the company which also owns the London Dungeons and Legoland were down 2.56 per cent i morning trading.