The mother of mentally ill Sarah Lawson, who died at her father's hands after a history of failed suicide attempts, appealed yesterday for improvements in mental health care in order to avoid similar tragedies in the future.

Karen Lawson spoke as Worthing priority care NHS trust announced an inquiry into Sarah's treatment and her ejection on the day before she died from the psychiatric unit where she had gone as a voluntary patient.

"Depression among young people is increasing. We need more and better facilities if tragedies like my family has endured are not to become commonplace," she said. "I want to appeal to those who are in a position of power to investigate the level of care mentally ill patients receive. Don't let there be another Sarah and another family traumatised by watching their child deteriorate before their eyes ... surely society cannot allow this to go on."

Mental health charities expressed their dismay at the case, but said that, horrifying as Sarah's death was, the only unusual aspect was that her father helped her die. According to a national inquiry, which took place earlier this year, there were around 250 preventable suicides a year.

"What we know from talking to young people is that services aren't meeting their needs in many cases," said Lesley Warner of the Mental Health Foundation.

"Sarah Lawson was turned away from hospital for allegedly using cannabis. Anybody with a dual problem - a mental illness coupled with drug use - is seen as untreatable by the mental health services. For many young people, the use of drugs or alcohol is sometimes a way of them trying to cope with a mental health problem."

"It is a tragic, terrible and unnecessary death that devastated a family, but it is not rare," said Sue Baker of Mind, the national association for mental health. "It is only rare for the family to be involved."

Its figures show that one in three people with severe mental illness are turned away from mental health services. "It's a national disgrace," she said, "and anywhere else in the NHS there would be an outcry."

Fifty organisations, including Mind, have come together under the umbrella of the Mental Health Alliance to push for changes in the Mental Health Act, which the government has promised to review. Two key demands are the legal right to a psychiatric assessment, followed by the correct care and treatment for any condition that is diagnosed. Mrs Lawson, who is separated from her husband but supports his decision to help her daughter die, said the family had not been able to get the right help for Sarah, 22.

"Getting the right treatment for Sarah proved impossible. At the very start my husband had to insist she be assessed by a psychiatrist, because her visits to the mental health hospital were once a week to see a nurse who suggested she take up a hobby," she said.

"This was a serious suggestion made to a young woman who made regular suicide attempts and lacerated her arms so deeply they needed suturing and later cosmetic surgery."

She had warned the Homefields mental health unit that it would be disastrous to send Sarah away. "I told them it was a dire situation and they would be giving her a death sentence if they discharged her. But still she was ejected out on to the street and six hours later Sarah was dead."

But the Worthing Trust which runs the hospital defended the decision to tell Sarah to go. "In common with all other voluntary patients, Sarah was asked to sign an agreement regarding the use of drugs and alcohol whilst in hospital which she did," it said in a statement.

"There was a serious breach of this agreement. Such breaches place other patients and staff at risk. An assessment was made by those responsible for her care, and given the nature of the breach, it was decided that Sarah had to be asked to leave the hospital."

It said that Sarah had been offered a comprehensive treatment plan. "We did our utmost to engage with her and her family over a long period of time," the statement said.