The cause of a factory explosion in which nine workers died was revealed for the first time yesterday.

A preliminary hearing at the High Court in Glasgow was told that the blast at the Stockline plastics factory in the city's Maryhill district happened after petroleum gas ignited in a pipe that had been corroding over the years.

The operators of the plant are accused of health and safety failures in relation to the 2004 blast, in which five men and four women died and 40 others were injured. Prosecutors said yesterday evidence relating to the deaths and the cause of the blast has been agreed with the defence in advance of next month's trial of ICL Plastics Ltd and ICL Tech Ltd.

Advocate-depute Angus Stewart, QC, told judge Lord Hardie the explosion was "caused by the ignition of petroleum gas from a pipeline".

Lord Hardie told victims and their family members in court that discussions between the Crown and the defence was normal in the run-up to trial and served to minimise the leading of evidence that was not in dispute.

Defence QC Paul McBride told the hearing that ongoing discussions with the prosecution could have a "significant impact" on the trial and could reduce its duration. He also said relatives of those killed and injured in the blast would not have to give evidence "about the tragic circumstances of that day".

The factory operators are being prosecuted under the 1974 Health and Safety Act.

The trial is scheduled to begin on August 13 and is expected to last around 12 days.