Jack Straw, the justice secretary, has ordered an inquest into 10 suspicious deaths at an old people’s convalescent unit in Hampshire.

The patients were among a group of 92 who died unexpectedly after being given abnormally large doses of morphine and other drugs at the Gosport War Memorial hospital. Their relatives believe their deaths were a form of euthanasia.

Straw has demanded the coroner’s investigation even though at least seven of the bodies were cremated. An inquest cannot take place in the absence of a corpse unless there are exceptional circumstances.

The justice ministry believes there is sufficient anxiety about the circumstances of the cases to require such a procedure, which, in the absence of remains, will be based only on a review of medical records and witness statements.

The allegation of “murder by euthanasia” is similar to that levelled against Harold Shipman, the GP from Greater Manchester who was Britain’s biggest mass killer. He was convicted of 15 murders but is believed to have killed about 250 of his patients. Shipman committed suicide in prison in 2004.

At Gosport, relatives complained repeatedly that the victims were not sick enough to require morphine. Questions about the hospital’s heavy use of the drug were also raised by the Commission for Health Improvement, the hospital watchdog.

Despite these concerns, police have been unable to gather sufficient evidence to pursue a prosecution. The two police investigations of the affair were themselves criticised for shortcomings.

The inquest into the 10 selected Gosport deaths was opened last Wednesday at Portsmouth and South East Hampshire coroner’s court.

A full hearing is scheduled for this autumn. A different coroner, Andrew Bradley, from Basingstoke, will conduct the process, which is expected to be the largest inquest of its kind.

The patients whose deaths are being investigated are Leslie Pittock, Elsie Lavender, Ruby Lake, Robert Wilson, Enid Spurgeon, Elsie Devine, Helena Service, Arthur Cunningham, Sheila Gregory and Geoffrey Packman. All 10 died between 1996 and 1999.

Ann Reeves, a beauty therapist, whose mother, Elsie Devine, 88, died in the hospital in 1999, has been one of the most vocal campaigners for the bereaved relatives. She is writing a book about the events and claims that questions had been raised as long ago as 1991 about the use of syringe drivers – automatic pumps that produce a continuous flow of morphine into a patient’s body.

“My mother was getting better until she went into that place. We are in no doubt there has been a massive cover-up. We are determined not to rest until we get justice for all of these patients,” she said.

Many of the other families are dismayed that their cases have not been selected for the inquest. Mike Wilson from Gosport says his 91-year-old mother, Edna Purnell, was out of bed and using a walking frame after a hip replacement operation, before she was transferred from Portsmouth’s Haslar hospital for a brief period of rehabilitation at Gosport.

“We have all her notes – we can prove what happened,” he said. “She was put to bed when she arrived there and given oral morphine, then transferred to a morphine pump. They threatened me with arrest when they caught me feeding her. They told me she was demented, which was not the case before they started giving her morphine. We are in no doubt that is what killed her.”

Richard Baker, a professor of clinical governance at Leicester University, carried out the statistical analysis that proved the abnormal scale of the death rate among Shipman’s patients. He is believed to have raised similar concerns about the death rate in Gosport.

The methods of at least two doctors and seven nurses working at Gosport have been subject to scrutiny. One of them, Jane Barton, a local GP who worked part-time at the War Memorial hospital, has been reported to the General Medical Council for unprofessional conduct.

A spokeswoman for Hampshire Primary Care Trust, which runs the hospital, said recommendations for improvements in the hospital’s practice had already been implemented. She pointed out the police investigations had come to nothing, and said the further scrutiny was “hugely distressing” for staff.

“We are confident [the hospital] provides safe, high-quality care,” she said.

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