An inquest jury's verdict that a 16-year-old non-swimmer who drowned during an outdoor activity trip was unlawfully killed was overturned today by the high court. A judge substituted a verdict of accidental death.

Herve Bola, from Woodford Green, Essex, died at the Sgwd y Gwladys Falls in Neath Valley, south Wales - known locally as Lady Falls - in July 2002.

Other teenagers on the trip claimed that a part-time youth worker with Redbridge council, Daniel Brown, then aged 21, had called to Herve to jump into a natural pool at the falls - a claim Mr Brown denied.

Today, Mr Justice Bennett, sitting at the high court in London, ruled that Neath and Port Talbot coroner David Osborne had made a mistake in law.

The judge said: "I have found that in this case this diligent and conscientious coroner has fallen into error and should not have left the unlawful killing verdict to the jury.

"Shortly and simply, the facts - taken at their highest - came nowhere near founding such a verdict."

Herve lived with his grandmother, Francoise Bola, 61, who attended court with his mother, Muzinga Bokwa, to oppose Mr Brown's application. Mrs Bola left in tears after the judge's ruling.

The group had been on an outward bound course at Glasbury House outdoor education centre in Hereford, which was operated by the London borough of Redbridge. The day they visited the Lady Falls, the group was hot and sweaty after taking part in abseiling and a zipwire activity and wanted to cool off.

Herve panicked and drowned within moments of leaping into the cold water. Efforts to rescue him were unsuccessful.

Other teenagers in the group said Mr Brown, who had been helping to supervise their activities and was in the water, had called out to the non-swimmer: "Jump in - I will catch you."

Staff members in the party all testified they had never heard Mr Brown call to Herve and said he had done everything possible to save him.

An instructor, Ian McLeod, also told the jury that Herve told him that day he was going to jump in the water, and had been warned not to.

But the inquest jury of nine, sitting at Neath county court, took three hours to reach their 7-2 majority verdict of unlawful killing after hearing almost seven days of evidence in April last year. They also visited the scene.

Quashing the unlawful killing verdict, the judge said it could only be seen by Herve's family, the media and public "as a verdict that Daniel Brown unlawfully killed Herve Bola".

The jury, therefore, had to be sure beyond reasonable doubt that Daniel had called to the non-swimmer: "Jump in - I will catch you", or words to that effect.

It also had to be "foreseeable" that the words would have led to the death, and Mr Brown's conduct was so bad that it amounted to gross negligence manslaughter.

He said Herve, who was about 1.8 metres (6ft) tall, had panicked and thrashed about when he hit the water, making it impossible for Mr Brown and others supervising the party of teenagers to save him.

The judge said: "The facts as found by the jury are wholly consistent with a verdict of accidental death."

After hearing that Herve's grieving family did not want a fresh inquest, he formally announced a narrative verdict of accidental death, saying: "Herve Bola, who could not swim, jumped into the natural pool in his response to encouragement from Daniel Brown who was expecting to catch him.

"Herve panicked on entering the water, and Mr Brown and other supervisors "were unable to maintain their hold on him. As a result, he drowned".

The judge expressed his deepest sympathy to Herve's family.

The judge found the coroner had been right to refuse to allow teenage witnesses to Herve's death to be cross-examined in the course of the inquest after they became abusive and aggressive to Mr Brown and other Redbridge council staff members.

Louise Christian, of solicitors firm Christian Khan, which represented Herve's family, said later: "Herve's mother and grandmother are extremely distressed that a high court judge has substituted a verdict of accidental death for the verdict of unlawful killing, which was delivered by a jury of ordinary people at the inquest. They continue to believe that Herve's death was due to an unlawful lack of care."