A sailor died 20 miles off the Norfolk coast after entering a ship's cargo tank unauthorised while the atmosphere was still toxic and lacking in oxygen, an inquest heard.

Ordinary seaman Ryane Palabrica was cleaning pyrolosis gasoline tanks on board a Stolt Skua when he entered the tank, which required the air in the tanks to be tested to ensure oxygen levels were sufficient for entry and permission given by the ship's chief officer.

His colleagues found the 26-year-old Filipino, who lived in Dingle, Iloilo, Philippines, lying on the first step in the tank on April 15, 2012, and he was airlifted to the James Paget University Hospital in Gorleston but did not survive.

Pathologist Dr Nathaniel Cary carried out a post-mortem examination, which revealed Mr Palabrica died from asphyxiation caused by the inhalation of benzene fumes. The inquest at Norwich Coroner's Court heard coroner's officer Mark Beresford read a statement from Angus McLean, principal surveyor with the Maritime Authority of the Cayman Islands, where the vessel was registered, which revealed Stolt Tankers had extensive procedures in place for entering the tanks, which met industry standards.

He said on the day of Mr Palabrica's death, the chief officer had addressed the ship's crew about the operation and the plan had been signed before the start of cleaning.

However, the ordinary seaman had apparently taken an individual decision to enter the tank, which was marked with hazard warning signs, after the Wilden pump he was using became stuck.

The statement said Mr Palabrica was wearing a filter mask, but this would only have offered him protection against toxic air if the oxygen levels in the air immediately surrounding him had been sufficient.Since the death, the company has changed its safety procedures, including banning the use of filter masks and providing deck crews with new breathing vests which offer protection in oxygen deficient environments.

The inquest heard Mr Palabrica had been on board the Stolt Skua since August 2011 and had worked for Stolt Tankers since 2006.

Jacqueline Lake, senior coroner for Norfolk, recorded a verdict of accidental death, adding: 'This is a tragic case of an ordinary seaman going into a tank with a face mask that was insufficient to protect him. I would like to offer my sympathy to Mr Palabrica's family.'