The mysterious death of a nine-year-old boy at Great Ormond Street Hospital for Children is being treated as murder, the police revealed on Tuesday.

David Stocker died from a heart attack on 20 August after an "abnormally high" level of sodium was found in his blood. The schoolboy, who lived in Hornchurch, Essex, with his parents, Keith and Patrina, had been in and out of hospital for almost two months while doctors tried to discover the cause of his illness.

Police are investigating whether the boy was poisoned some weeks before he was taken to hospital. His death is not thought to be linked to treatment he received.

The high level of sodium found in the boy's blood was not present because of any illness or from natural causes, an inquest into David's death was told yesterday.

Detective Inspector Chantal McMurdie told St Pancras Coroner's Court in London: "The family are aware that we are carrying out extensive inquiries. It is a murder inquiry."

David was admitted to Oldchurch Hospital, Romford, on 21 June with a number of symptoms, but the medical team was unable to establish what was causing the illness. He remained there for four weeks before being sent home on 28 July.

He was admitted again three days later after falling ill, and the following day was transferred to Great Ormond Street Hospital in central London. He suffered a heart attack there on 18 August and went into a coma. His life-support machine was switched off at 11.05am on 20 August.

After the boy's death, the Metropolitan Police's serious crime group launched an investigation. Officers are still awaiting the results of tests from the hospital and the post-mortem examination.

The St Pancras coroner, Dr Susan Hungerford, formally opened the inquest and adjourned it to a date to be fixed while police continue their inquires. No family members were present.