The parents of a five-year-old girl who died at Oxford's John Radcliffe Hospital during an operation to remove her spleen are launching a medical negligence claim.

Bethany Bowen, pictured, had a vital artery ruptured by a surgical instrument, a coroner in Oxford ruled today.

Bethany, from Cricklade, near Swindon, started bleeding through a tear in her aorta last July and, despite attempts to resuscitate her, she died less than a hour later.

A spokesman for Oxford Radcliffe Hospitals NHS Trust said last night that the trust had "apologised for the terrible outcome of this operation" and admitted legal liability.

The spokesman added: "We investigated the circumstances of the death in great detail, and sought independent expertise to try to help us to understand what went wrong. It is not possible to say with any certainty what caused Bethany's death."

Paul Rumley, a clinical negligence specialist at Withy King Solicitors, said: "Mr and Mrs Bowen are disappointed with the verdict. They still maintain that the operation performed on their beloved daughter Bethany was not the one they consented to. They still earnestly wish that the lessons from Bethany's tragic death are learned not just by this trust, but by all hospitals and surgeons operating in this field."

Oxfordshire assistant deputy coroner Dr Richard Whittington said that the fatal injury was caused during her surgery.

However, he said it seemed unlikely that a new device, a morcellator, being used for the first time by the surgeon, was responsible.

Surgeons were using the morcellator - a rotating scalpel - to break down the spleen, which was surrounded by a plastic bag, before its removal.

One of the three surgeons on the team had experience of the device but the doctor using the instrument had seen it for the first time that day. The girl's parents said they had no idea the device was going to be used.

But the inquest heard that the surgeon had received the necessary training.

Gordon Mackinlay, consultant paediatrician at Edinburgh's Royal Hospital for Sick Children told the inquest the instruments used in the rest of the keyhole operation, the graspers and retractors, had probably caused the damage.

Recording a narrative verdict, Dr Whittington said the girl had died as a result of surgical trauma caused by "an unspecified surgical instrument".

Dr Whittington said Mr Mackinlay's suggestion was "just a theory, but a plausible one".