An inquest into the death of 15-year-old Pippa McManus found the private hospital also failed to organise proper aftercare for the teenager, who took her own life.
A jury at stockport Coroner’s court heard how on the day she died, Pippa had argued with her family over excessive use of the gym and warned she was going to kill herself.
The tear in the thread of our family will never be mended
The court found staff at The Priory Hospital Altrincham had developed “no relationship” with people who could have help safeguard the teen once she was released, despite having cared for her for a year.
Yesterday her parents said failings in their daughter’s care were “quite disturbing” and had been responsible for her death.
Following the argument in 2015, Pippa walked away from the family home in Frances Avenue to Gatley station, where she stepped in front of a train.
The jury criticised the planning for her discharge, and found there was "inadequate engagement of community support" for Pip's family and a "failure to implement a care plan in a timely manner".
They did not, however, criticise the decision to release her itself.
The Priory is one of the UK's largest mental health providers and has multi-million pound contracts with the NHS.
Last year the mother of 14-year-old Amy El-Keria, who died from neglect at a Priory facility, to lose the contracts.
Speaking outside court, Pippa's mother Marie said: "We believe the failings of our daughter's care, from beginning to end, resulted in her death.
"The tear in the thread of our family will never be mended. Pip spent her last three years fighting against against anorexia, malnutrition, depression and self-harm.
"Too many of our children are dying from this terrible illness [anorexia]. Effective treatment is needed more quickly, and if this had been available to out beautiful daughter, maybe she would still be alive today."
The teenager’s father, Jim McManus, said his daughter had been “failed” and describe some of the care she had received as “quite disturbing”.
“It's not the ending we want, however the jury came up with the right answers,” he said.
"It was quite deflating to see all the hard, goodwill, for her to build her weight back up - she desperately wanted to get her life back on track - and the illness would suddenly, at a flick of a finger, pull her back down to earth again.
"It wasn't pleasant at all. She was a lovely girl, plenty of fun. Always active."
Paula Stanford, Hospital Director of the Priory Hospital Altrincham, said: “Our heartfelt sympathies are with Pip’s family and we will now carefully consider the findings of the jury.”