A North-east chemical worker died, overcome by toxic fumes while preparing to handle acid.

Production technician Derek Tuck, 41, collapsed at the BASF plant, at Seal Sands, near Billingham, Teesside, and was pronounced dead on arrival at hospital.

The first day of a three-day inquest at Middlesbrough heard that an internal investigation found that the father-of-three had probably been carrying out an unauthorised procedure.

A jury heard that each day, two-man teams took samples of stored, but volatile, hydro-cyanic acid for laboratory tests. Company procedure laid down that a technician wearing breathing apparatus and face mask would take a sample only in the company of a colleague on standby in case of an emergency.

The jury was told Mr Tuck was due to take a sample when his standby man received a call to assist what Health and Safety Executive inspector Peter Hanlon called an "inexperienced" operator elsewhere on the plant.

He returned to find Mr Tuck on the ground, his breathing apparatus on his back, but his protective face mask on his chest.

BASF director Dr Anthony Birch said: "On the balance of probability the following scenario is believed to be the most likely: Derek Tuck was making an unauthorised preparation for sampling.

"He did this without donning his face mask. No standby man was in attendance."

Dr Birch said it appeared Mr Tuck could have opened an isolation valve on a flare tank but did not make sure a needle valve controlling the flow of hydro-cyanide was fully closed. As a result, he was overcome by fumes following a sudden discharge of the process fluid.

Dr Birch said this was in breach of company safety procedures which, if discovered earlier, would have resulted in disciplinary action against Mr Tuck, of Tallgarth Road, Norton, Stockton.

The inquest heard that the HSE, which investigated the incident, on December 20, 1999, ordered BASF to take immediate action, backed by enforcement notices detailing work to be carried out.

The hearing continues.