A mechanic poisoned by fumes after a builder accidentally blocked a boiler pipe while re-building his chimney was unlawfully killed, jurors ruled today.

David Johnson, 56, of Ipswich, Suffolk, was guilty of the manslaughter of Robert Schenker, a jury concluded after a two-week trial at Norwich Crown Court.

Sentencing was adjourned until May 19. Legal sources said Johnson could be jailed for several years.

Mr Schenker, 31, of Peterborough, Cambs, died of carbon monoxide poisoning after Johnson blocked the flue leading from his kitchen boiler, jurors were told.

Prosecutors said Johnson had been "grossly negligent".

He had allowed mortar to drop and drip into the chimney and the mortar had solidified, preventing carbon monoxide from escaping into the atmosphere. The gas had leaked into Mr Schenker's house and killed him.

Mr Schenker, who ran a business selling spare parts for Porsche cars, was found dead in bed in March 2006 - three days after Johnson started work, jurors heard.

Johnson, who lived in Peterborough when Mr Schenker died but now lives on a boat in a marina at Ipswich, had denied manslaughter.

Johnson was prosecuted following a joint investigation by police and health and safety inspectors.

"This has been a very difficult, complex and unusual case," said Detective Inspector Mick Birchall.

"We hope this result will serve as a warning to other construction workers to make sure they comply with all health and safety regulations."

Paul Hoskins, an inspector with the Health and Safety Executive (HSE), added: "Robert Schenker's tragic and wasteful death could and should have been avoided by Mr Johnson taking straightforward safety precautions.

"HSE inspectors involved in this case were appalled by the poor workmanship and failure to carry out even the most basic checks to ensure the correct operation of the flue, which directly led to the fatal incident."