Mrs Purnell was being taken home from this day unit at Barry Hospital

Mary Purnell, who had dementia, fell after being left at a house in Dinas Powys, near Cardiff, by an ambulance crew. Barry magistrates heard that she died weeks later in hospital.

The two trusts admitted failure to discharge duty of care by exposing patients to serious risk of injury.

Sentence will be passed next month.

The trusts later said they had reviewed their practices and "apologised unreservedly" to Mrs Purnell's family.

The court heard how the widow was picked up from a day centre at Barry Hospital and mistakenly taken to the home of another pensioner in Dinas Powys in August 2003. Her actual home was five miles away in Penarth.

She was left alone in the house by the ambulance crew, who found a door key under the doormat and let her in.

The failure of the duty of care exposed Mrs Purnell to serious injury and contributed to her death Proescutor Garth James

In her effort to escape, the court was told, Mrs Purnell broke a leg and died five weeks later in hospital from bronchial pneumonia and leukaemia.

Prosecutor Garth James said the regular ambulance drivers were away and a replacement crew took over, ferrying patients from the hospital day care centre.

"Both crew were unfamiliar with the passengers and the route," Mr Thomas said.

"One of the passengers had been taken home by her son but was still on the list on the bus. The driver assumed Mrs Purnell was this passenger and drove her to the unfamiliar house.

"The driver found a key under the doormat and sat Mrs Purnell down in the front room of the house."

This was a tragic incident that should never have happened. The family are pleased that matters are drawing to a conclusion, and that there is now the opportunity for some closure Purnell family statement

The court heard that the other passengers protested but the crew carried on regardless. The error came to light some hours later and Mrs Purnell was found injured, having fallen in the garden of the house.

Mr James said: "The failure of the duty of care exposed Mrs Purnell to serious injury and contributed to her death."

A statement on behalf of Mrs Purnell's family read: "Today's admission of guilt by the trusts involved is welcomed.

"This was a tragic incident that should never have happened. The family are pleased that matters are drawing to a conclusion, and that there is now the opportunity for some closure.

"They will obviously never forget Mrs Purnell or the events of that time but they hope that lessons have been learnt so that others will not have to go through such a horrendous ordeal."

The case was passed to Cardiff Crown Court for sentencing on 7 April.

District judge John Charles said that there had been "serious shortcomings" and due to the serious neglect involved, the matter should be dealt with by the crown court.

An inquest was held in January last year in which Cardiff's coroner recorded a narrative verdict on Mrs Purnell's death.