Twenty-four-year-old Sam Foley, who had been helping out at a friend's farm at Snetterton in Norfolk, was attempting to tip manure on July 8 last year when the trailer jack-knifed and its tow ring broke.

Inspectors from the Health and Safety Executive and other experts concluded that the accident was caused by the tow ring – which would have cost less than £100 to replace – snapping in two after it had been welded following a previous fracture.

The type of trailer used was also unsuitable for the load of solid cargo and the fact its tailgate was locked would have put extra strain on the tow ring because the manure could not escape when the trailer tipped.

The inquest jury at Norwich heard that Mr Foley, of Windsor Road in Newmarket, had helped Pearn Wyatt & Son for eight years on a casual basis.

Father and son Timothy and Jonathan Wyatt claimed to have no knowledge of any repair work and said they would always replace damaged tow rings.

The Wyatts fitted the trailer to the tractor at about 8.20am on July 8 and, giving evidence, Jonathan Wyatt said Mr Foley locked the tailgate before leaving the yard because he was "very conscientious".

No-one witnessed the accident but the Wyatts became concerned and Mr Foley was discovered at 8.50am.

Health and Safety Executive inspector Malcolm Crowther, who attended the scene, said the trailer was 26 years old and in poor condition with signs of damage and corrosion.

"The tow ring had previously fractured on one side and an attempt had been made to do a weld repair," he said. "The result of the repair was inadequate and failed, resulting in the fatal accident."

He added that the repair "seriously compromised" the strength of the tow ring so that it had only 45 per cent of its original strength on one side.

Jonathan Wyatt said it was possible that the tow ring could have been repaired while on loan within the local farming community.

Timothy Wyatt added: "It is a complete mystery. When something needs replacing, it's replaced and done properly."

The jury returned a verdict of accidental death following instruction from Greater Norfolk Coroner William Armstrong.

Mr Armstrong added: "It's a matter of grave concern that this tragic death occurred as a result of, principally, the tow ring failing and that some person unknown attempted to repair a previous fracture by welding, which should not have been done."